I am not sure that this is a DBMail bug. I'm having the same problem with Mail.app and non-dbmail imap servers (cyrus/uw-imapd). Thisleads me to believe this is a Mail.app bug (and it is bloody annoying).
I'm pretty sure the problem was added when upgrading to 2.2.0 from an older 2.1.7. I did have problems deleting messages before, which magically returned after switching folders, but that problem is gone now :)
Marc