As quite a few googlers appear to read and write on HN, I'd really welcome an insider info on what's going on the last few days. Sure there will be some internal turmoil going on right now, but isn't there some non-confidential info to share? Can't imagine this will hurt the image of google neither in the short nor long run, quite the opposite.
I wonder if Gmail is just not a very well maintained codebase. Here's an issue where old emails just become inaccessible. 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. https://support.google.com/mail/thread/6187016 Maybe time to switch to a more reliable provider.
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.