It's an artifact of IMAP having evolved in the presence of older mailstore technologies like mbox and maildir that couldn't easily accommodate long-term stable

It's an artifact of IMAP having evolved in the presence of older mailstore technologies like mbox and maildir that couldn't easily accommodate long-term stable message IDs. It's fine though, as long as the MUA developers understand this and don't expect UIDs to be more stable than they actually are.