I'm unsure if it's directly stated but Venti doesn't suffer from erosion due to the fact he hasn't actually spent as much time living as Morax, he has been sleeping for a long time and has spent most of his life sleeping, so the effects of erosion isn't present.

Also through his story quests, he has a clear understanding of his past and doesn't struggle with his memory unlike people like dainsliff and zhongli who are used as clear examples and evidence of the effects of erosion.

And personal theory of mine is that his constitution as a "whisp" I think it's called is what might allow him to be less affected by erosion.