It's a new theory proposed by modern scholars of Vālmīki Rāmāyaṇa based on textual criticism. As noted by Robert and Sally Goldman in their translation of Uttarakāṇḍa, the same traditional commentators who wrote commentaries on the previous kāṇḍas have also commented on Uttarakāṇḍa. So, it's unlikely they were aware of problems with the Rāmāyaṇa text passed down to them, enough, to question the authenticity of Uttarakāṇḍa.
Compared to the commentators' intense interest in virtually every
aspect of the first six books of the Rāmāyaṇa on which they often
write dense, lengthy, and not infrequently contentious essays on such
matters as theology and chronology, and frequently propose complex
reading strategies involving what they see as the manifest
(spaṣṭārtha) and latent (vāstavārtha) meanings of the text, their
attention to the bulk of the Uttarakāṇḍa tends to be relatively
scattered and episodic. Rarely do they write at any length at all, and
when they do, it is mainly to reprise previously debated matters such
as the two just mentioned. Some of the commentators, such as
Maheśvaratīrtha (Cm), who had taken quite active roles in the earlier
debates, appear to drop out of the discussions of large sections of
the text or, at any rate, are no longer quoted at length by the
editors of the various editions in which their commentaries appear.
Moreover, Mādhava Yogīndra (Ck), Govindarāja (Cg), and Nāgeśabhaṭṭa
(Ct), who in the previous books had often had quite differing views,
here frequently say more or less the same thing on a given verse as if
the later scholiast were merely echoing the former. Even the somewhat
eccentric Satyatīrtha (Cs) who, in his comments on the earlier kāṇḍas,
had delighted in extensively quoting and then ridiculing his rivals
"Bhaṭṭa" (i.e., Nāgeśabhaṭṭa [Ct]) and "Tīrtha" (i.e., Maheśvaratīrtha
[Cm]) now mentions the former only occasionally and the latter more
sparingly still. The previously equally contentious and independent
minded Mādhava Yogīndra (Ck) now exercises himself primarily in the
case of passages that directly address Rāma's return to his primordial
divine form as such passages allow him to return, often at
considerable length, to his preoccupation with his Brahmaite
theology. What can explain this relative lack of interest?
The answer to this question is to be found, we believe, in the entire
problematic of the Uttarakāṇḍa as it has often been viewed by Indian
audiences. For one thing, as we have discussed at length above in our
section on the controversial nature of some of the book's best-known
episodes, many modern audiences of the epic have been disturbed by its
hero's actions in killing the śūdra Śambūka and banishing Sītā. Thus,
although the modern reader might well expect these portions of the
kāṇḍa to draw the greatest critical attention and rationalization on
the part of the commentators, in fact it appears that they
overwhelmingly prefer to pass over them lightly, avoiding the expected
moral questions to focus on secondary issues or to radically
reinterpret the substance of the narrative itself. In this way, as we
have shown, the commentators seem utterly unconcerned with Rāma's
execution of Śambūka as having any moral significance other than its
success in restoring the brahman child to life. Instead, they show
interest here only in a discussion of the progression and character of
the yugas, which make the śūdra's penances a crime. In the case of
Sītā's scandal and banishment, most of the commentators again tread
lightly with the exception of Śivasahāya (Cr), who deploys his
ingenuity and the resources of the Sanskrit language to argue that no
such things actually occurred.
The first major work by a modern scholar to critically examine the Rāmāyaṇa was probably Hermann Jacobi's Das Ramayana. From the foreword to the English translation by S. N. Ghosal:
Dr. H. Jacobi's German work, Das Rāmāyaṇa (Bonn, 1893), is a very
important landmark in the progress of Rāmāyaṇa studies. Coming from
the pen of a great Indologist, it is useful to all the
research-workers in this particular field more than even six decades
after its publication. The work was of special interest to the
Oriental Institute, on account of its project of the Critical Edition
of the Vālmīki Rāmāyaṇa. Dr. S. N. Ghosal had prepared, at the request
of the Director of the Institute, an English translation of the German
book, and the same was published in installments in the Journal of the
Oriental Institute (Vol. V, No. 2 to Vol. VIII, No. 3)...
It is hoped that the publication will be useful to all interested in
Epic studies in general and Ramayana studies in particular.
We are thankful to Dr. Ghosal for translating this work which was not
accessible so far to many scholars not conversant with German.
B. J. Sandesara
Director Oriental Institute, Baroda
June 30, 1960
Commenting on Bāla and Uttara kāṇḍas, this is what Jacobi says:
In this way the Rāmāyaṇa gradually swelled in bulk and would have totally fallen asunder had not a corpus been ascertained. The purpose was served no doubt by the index of contents that occurs in the first canto of the First Book. Since the content of the First and the Last Books is not alluded to here, so the ascertaining of the corpus must have taken place before the origin of the First and the Last Books. Probably the Diaskense contains not only the original portion of the Rāmāyaṇa, but also its divisions into the sargas. Because in the Uttarakāṇḍa reference to sargas has been made on many occasions.
As elsewhere the continuous process of writing additional verses by the rhapsodists was not restricted to the contents of the original poem, but was also extended to those portions, which were not actually treated by the first poet: it is what remains at the beginning of the story, narrated by the first author—namely the prehistory of the heroes and his rivals (the beginning of the romantic epic) and the continuation of the original story. It thus actually happened in India. The youth of Rāma is sung in Bālakāṇḍa and his story is continued right up to his death in the Uttarakāṇḍa. Here the necessity was felt to stabilise the inflated epic corpus. An evidence in support of this fact is supplied by an index of contents of the 3rd canto; nevertheless the Uttarakāṇḍa does not seem to have obtained its final form then, because its content is very broadly mentioned in the 3rd canto. It is indicated also by VII 94, 26.
ādiprabhṛti vai rājan pañcasargaśatāni ca |
kāṇḍāni ṣaṭ kṛtānīha sottarāṇi mahātmanā ||
The Bāla and the Uttarakāṇḍa provide evidence in support of the fact that a long period elapsed between the composition of the same and that of the original epic.
Through this very work the hero of the Rāmāyaṇa became converted to the ethical hero of the people and from the hero of a clan to a national hero. The honour, apportioned to him, raised him forthwith from the human to the divine sphere and brought about his identification with Viṣṇu exactly in the same manner as occurred with regard to Kṛṣṇa, another epic hero of Western India; and it could scarcely be otherwise in euhemerism as referred to by Sir Alfred Lyall in concern with the evolution of the Indian religion. In the case of both Rāma and Kṛṣṇa a legendary hero has been mingled with a popular god: Kṛṣṇa, the Yādava with the shepherd god Govinda and Rāma, the Rāghava with the national god Rāma, the conqueror of the demons. Just after this happened the so-conceived god was assumed as an incarnation of Viṣṇu.
The deification of Rāma and his identification with Viṣṇu are such notable facts in the First and the Last Books, that they loomed large before the vision of the poet. But in the five genuine Books this idea cannot be traced barring, of course, a few interpolated passages; contrarily there Rāma is out and out a man. It certainly required a longer time before the transformation in Rāma's character, that appears in both the later-added books, had been completed.
We are led to the same conclusion by the fact that in the First and the Last Books Vālmīki appears already as a contemporary of Rāma and as a revered Ṛṣi. Both had been possible, only then when Vālmīki had remained far in the distance from the later poets, as a result of which the shroud of mythic glory wrapped him and made him obscure to them. We cannot approximately ascertain the time, that was necessary for the purpose; but it is certain that it is to be measured rather by centuries than by decades. That the Rāmāyaṇa was still open for admitting additional passages since the Greeks and Scythians were known to the Indians, will be shown in § 4.
The Rāmāyaṇa in its present shape is the outcome of a long pursuit of epic composition. Yet the tradition, which sees in it a homogeneous poem, is maintained though with certain restriction. The nucleus of the poem, round which the epic composition of many generations has been gathered, was certainly a homogeneous work of an eminent poet.