> Not fixed for over a year and they've locked the thread so I'm starting to wonder if they actually deleted the emails by mistake.

Did you try pulling them down using the API tester?: https://developers.google.com/gmail/api/reference/rest/v1/us...

Some of the internal formatting that Gmail uses has changed over the years, so more likely than not the API that parses the stored message for display in the Gmail UI is just throwing some kind of error.

I didn't but I did try Takeout and they weren't in it.

Either way my point is that this is a pretty serious bug and they haven't even acknowledged it! Not a good look.

I've never had issues over IMAP with old (decade) message in gmail

Right but the version of an email message you download via IMAP is different than the version of an email message you see in the Gmail UI. That's my point, that the error is probably in the way Google is processing messages for Gmail, so you wouldn't see it in IMAP or via the API.