You make a good point but this is not a "strange assumption" by any stretch. You see Thunderbird has a "move" action. It allows you to move emails from a folder to another. Now somehow, this is apparently NOT implemented as:

1. Copy from source to destination

2. Verify copy has completed without issue

3. Delete from source

but... some other way? So when you try to move from a server folder to a local client folder by an innocent looking drag an drop, combine this with a poor network connection, a garbage tier legacy protocol like IMAP and decade old C++ spaghetti code, and you get this textbook 17 year old severity 1 bug that will never get fixed.