IMAP is a really broken protocol; I'd characterize it as a database synchronization protocol that wasn't designed to be one. You need to rely on certain extensions to have a hope of being bug-free [1], and even those can be problematic (UIDVALIDITY changed, gotta clear your entire inbox!).

[1] The IMAP protocol used to rely heavily on message sequence numbers, where each message in the folder was numbered sequentially from 1-N, and deleting messages caused all subsequent messages to have their number decremented by one. But you can have multiple connections active on the same mailbox, and you can't always immediately notify the other connections of when things get deleted, so every connection can have a slightly different mapping. So everyone uses UIDs now, which are guaranteed to be unique, until the server feels like deleting them all and restarting from scratch (i.e., updating UIDVALIDITY), so every message has a different UID now.