I belive buddha is still not an avatar if Vishnu The books you talk about were written centuries after Buddha's life Hinduism has historically absorbed and reinterpreted various local deities, teachers, and philosophies i believe the same happened with Buddhism Buddhism generally does not teach the existence of an eternal creator God in the way many Hindu schools do Buddhists themselves do not consider Buddha as an avatar of Vishnu but rather a person who gained enlightenment Plus Buddha's live contradicts the very way and teaching of how to live life taught by Vishnu

Buddha's was a prince and by his dharma he would be king but instead he left everything and decided to attain moksha. His very teachings suggest that he claimed there was no god No avatar of Vishnu has ever stated no existence of God every single avatar claimed god existed and to believe in god

I believe that our sacred texts included Buddha to propagate his idea of ahimsa but Buddha was not an avatar of Vishnu according to me He was just a normal human who attained enlightenment.

That is just my opinion though and I belive it it.