Earlier quoted context omitted.
I had email service expire on a domain and moved it to fastmail. Fastmail, obviously, did not have copies of the email I received before the move. But my phone did. 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, b…
I'd wager that this is marked as "by design" because it technically follows some IMAP spec. What the app does, purely on a technical level, is correct. I've so often had debates and threads in issues where a developer entrenched in a domain has so much domain-knowledge (ie. tunnel vision) that "technically correct" or "by the spec" is the only correct way. I've been that developer in many cases too, in hindsight.
Another example is HTTP Strict-Transport-Security which specifies that browsers should take control away from users.