I can't say I've encountered this problem, though I haven't had an opportunity to sneak attack as an AoO and manage to meet sneak attack requirements. When I take control of Astarion sneak attack only procs once as per 5e rules without needing to select it. I assume it's a toggle under passives?
Sneak attack is on the "Reactions" tab when you open the character sheet. (Like Divine Smite and some other things that aren't
really reactions, it uses a variation on the same mechanic as reactions.)
I definitely have seen Sneak Attack trigger on AoOs, but I can believe that's only when my character hasn't already used their Sneak Attack that round.
