Note that this is why you use copy, check, then delete, instead of just "move" data, whenever it's important that the process works correctly
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)
But OP said it also deletes the message in the server, so it may be a different bug, right?