When I updated the configuration in K-9, it contacted the fastmail server, found that the mail it had locally wasn't also present there, and immediately deleted all my local copies.
That's not a "bug" in the sense of unintended behavior of the software, but it certainly seems like the software designer's goal is to hurt the user. I obviously didn't want that to happen. There is no scenario where I would want that to happen.
Email clients make some strange assumptions about what kind of actions make sense under what conditions.