It is traditionally accepted that adhyatma Ramayana is a part of the brahmanda purana but according to Wikipedia it was written in 14th century.Does that mean it was added later to the brahmanda purana? This website here https://www.wisdomlib.org/hinduism/book/the-brahmanda-purana Doesn't include adhyatma Ramayana in the Translation.Matsya purana Chapter 53 says that brahmanda purana consists of 12,200 verses.If we include adhyatma Ramayana,the number of Verses excede 18000.
Scholars ascribe Adhyatma Ramayana to the period 14th-15th century.
Though depicted as a part of the Brahmanda Purana, respected as a canonical work of the Vaishnavas and begun in the Tantra style of conversation between Siva Mahadeva and Parvati, scholars ascribe it to the period 14th-15th century, A.D. Nothing is known of the author, though the Rama tradition believes it to be Vedavyasa since it is said to be an integral part of the Brahmanda Purana.
A Concise Encyclopedia of Hinduism by Swami Harshananda
1Do not conclude anything like that just by seeing answers posted on this site. Here real Pundits or experts don't answer. Just go by what is traditionally followed and accepted @RishikeshS One answer containing few lines can not dismiss a scripture as inauthentic just like that. Jul 23, 2020 at 13:42
1@RishikeshS I don't know if the book is on line. I just bought the 3-volume Encyclopedia a month ago from Amazon and am studying it. Jul 23, 2020 at 23:14
2I don't have the power or expertise to prove a scripture authentic or inauthentic. Just note that only sceptics (Indologists) try to date Hindu scriptures because they think, like any other books written by humans, scriptures also must have a date. They rule out divine involvement in the existence of the scriptures. According traditional Hindus, scriptures can not be assigned dates. Now, it's upto individuals what to believe and what not to. @RishikeshS Indologists are not experts in the context of religion, spirituality or Hinduism. Jul 24, 2020 at 6:33
3Apart from the 20,000 + mantras of the Vedas that had to memorized, most scholars claim that many Vedic texts have been edited and rewritten. Sometimes an entire text has been inserted in other texts. A famous example is Markendaya Purana's Chandi, the scripture of Saktas, which is considered to be an interpolation. There are even quotes from Rig Veda that has not been found in the current version of Rig Veda. Most Hindu scriptural texts have come down to us in altered form. At least that is what scholars claim. Jul 24, 2020 at 11:33
most scholars --- Those scholars you are talking about are Indologists, sceptics. In matters related to religion their opinion has zero value. Those who are incorporated into Hinduism by Samskaras (like initiation) and have Sadhana (practice) of some magnitude are only entitled to give any opinions about Hinduism that are worth listening to @PradipGangopadhyay Jul 24, 2020 at 18:06