p.s. It makes sense that the poll would reset in order to prevent abuse, but the problem here is the lack of warning and the inconsistency.
You can edit the main part of the post without resetting its poll, which gives the impression that editing doesn't reset anything. The reset only happens if you change the poll options themselves.
It would be nice if Mastodon had a confirmation dialogue box ("This will reset your poll, are you sure?") if you try to edit the labels.