After a user replied to my comment in a thread, their own previous comment from a week ago disappeared, and my comment that they were replying to now shows as being from this other user, at a different time, and it says it was edited (I never edited it and they can't edit my comments).
The time it now says the comment was made at is a time when nobody made a comment.
I don't think I can raise a bug with Jira Cloud, so reporting it here.