Never, I don´t need to farm comments for votes, I only comment when I want to comment.
However, I have in the last time increasingly the suspicion that some comments on my posts are indeed AI-generated.
Red flag 1: Comments are in German despite the author of the comment is not even German-speaking.
Red flag 2: Smooth elaborate english from non-native English speakers.
But as you say, there is not much one can do about it, it is not easy to prove it.