This is what I would expect. If it is e2e, how would Meta decrypt your old messages and reencrypt them? I guess it could do it client side. But that would be kind of crazy
Since I only experimented with/created secret chats in the past with close friends and family, all my old conversations with years of history were archived with no path to restore.
This is a horrible UX experience and absolutely ridiculous that this design passed in Meta.