This is what Mahabharata says:
Unto the cheerless and trembling Karna, prostrated with joined hands
upon earth, that foremost one of Bhrigu's race, smiling though filled
with wrath, answered, 'Since thou hast, from avarice of weapons,
behaved with falsehood, therefore, O wretch, this Brahma weapon shall
not dwell in thy remembrance. Since thou art not a Brahmana, truly
this Brahma weapon shall not, up to the time of death, dwell in thee
when thou shall be engaged with a warrior equal to thyself! Go hence,
this is no place for a person of such false behaviour as thou! On
earth, no Kshatriya will be thy equal in battle.' Thus addressed by
Rama, Karna came away, having duly taken his leave.
Mahabharata Santi Parva Section III
This is what happened at the time of Karna's death.
'At that time, when hour of Karna's death had come, Kala, approaching
invisibly, and alluding to the Brahmana's curse, and desirous of
informing Karna that his death was near, told him, "The Earth is
devouring thy wheel'. Indeed, O foremost of men, when the hour of
Karna's death came, the high Brahma weapon that the illustrious
Bhargava had imparted unto him, escaped from his memory.......When his
car began to reel from the curse of the Brahmana and when the high
weapon he had obtained from Rama no longer shone in him through inward
light, and when his terrible snake-mouthed shaft also had been cut off
by Partha, Karna became filled with melancholy.'
Mahabharata Santi Parva Section XC
No definitive answer can be given to this question. One possible answer is to say that Arjuna+Krishna were superior to Karna in battle thus violating the 'equality in battle' condition. This weakened the effect of Parashuram's curse.
Of course one may object and say that Krishna stayed passive throughout the battle against Karna. I would say that Krishna's presence was vital throughout the Mahabharata war. Just think of Arjuna's plight after Krishna's death. In fact Krishna himself hinted that he was not entirely passive during the war.
Krishna’s defense Krishna defended his actions as follows in a voice
deep as that of the clouds or the drum: "All of them were great
car-warriors and exceedingly quick in the use of weapons! If ye had
put forth all your prowess even then ye could never have slain them in
battle by fighting fairly! King Duryodhana also could never be slain
in a fair encounter! The same is the case with all those mighty
car-warriors headed by Bhishma! From desire of doing good to you, I
repeatedly applied my powers of illusion and caused them to be slain
by diverse means in battle. If I had not adopted such deceitful ways
in battle, victory would never have been yours, nor kingdom, nor
wealth! These four were very high-souled warriors and regarded as
Atirathas in the world. The very Regents of the Earth could not slay
them in fair fight. Similarly, the son of Dhritarasthra, though
fatigued when armed with mace, could not be slain in a fair fight by
Yama himself armed with his bludgeon! Ye should not take it to heart
that this foe of yours hath been slain deceitfully. When the number of
one's foes becomes great, then destruction should be effected by
contrivances and means. The gods themselves, in slaying the Asuras,
have trod the same way. That way, therefore, that had been trod by the
gods, may be trod by all."
(Mahabharata, Salya Parva, Section 61)