Thunderbird auto-updated from 78.14 to 91.1.1 yesterday. I'm using Thunderbird to access my Gmail account via IMAP. Everything was working fine in version 78, but now when I open a new message, the last deleted message reappears in the inbox. If I have multiple new messages in my inbox, read one and delete it, as soon as I open the next message, the one I just deleted will reappear. Sometimes deleting the message that reappeared causes the another one to reappear, and I have to keep deleting them again until they stop. For example, just now I had two unread messages when I started up Thunderbird. I read the first one and deleted it. When I opened the second one, the first one reappeared. When I deleted the second message, and then re-deleted the first message, the second message reappeared. Deleting second message one caused the first message to reappear again. This went back and forth several times before they stopped reappearing.