This seems to be fairly recent change. According to the bugzilla docs it's called something like `commenton*`. This is really annoying because the UX is not nice. If Bugzilla had a "Close and Comment" button it would be less annoying, but since it doesn't, when you comment on a bug before changing the status, and then change the status, it makes you put a junk comment to allow actually changing the status. IMO, this setting, given the UX of bugzilla, is a bad idea. I've seen at least a few times other people having to put a dummy comment to change the status.
Most recent example of mine: https://bugzilla.xfce.org/show_bug.cgi?id=12137 It also doesn't help that Bugzilla (or a setting?) doesn't let you delete the junk comment.
commentonchange_resolution is now set to off, and this bug should be fixed.