I have no good alternative to thunderbird, it does so much of what I want. But this bug is awful.
But OP said it also deletes the message in the server, so it may be a different bug, right?
Even if the software doesn't have known bugs, I do it if the data is important enough and especially if I were to not have a backup (for example, because the storage provider takes care of backups and redundancy. I personally like to have another copy that I manage myself, but how many people have their IMAP emails or Spotify playlist data backed up for example? I do, but not many people I think)
As a software developer fixing stuff like this is only possible if you can reproduce it or otherwise get logs, telemetry and similar things, otherwise it is pretty much just guesswork.
Granted given the severity of the consequences I would've chosen a more defensive move-strategy (e.g. one that deletes mails only once they have been copied verifiably), but that would have significant performance impacts in the 99.99% of cases where it works, so finding the real problem is preferable.
The truth is that if this happens to you regularly, that you are probably the prime person to gather more data on this. Call it giving back to Open Source software.
Same but I never move my emails to local folders.
As a software developer you should be able to reason about your code and work backwards from the observed result to investigate possible causes.
When a plane crashes or bridge collapses the engineers tasked with finding the cause don't just throw up their hands if they can't make it happen again.
But this does seem to be a different issue from the one OP mentioned. Seems to be actually this one: https://connect.mozilla.org/t5/discussions/thunderbird-searc... (with suggested solution)