Well, some dirty cronjob could periodically expunge all emails marked as DELETED from a specific user. You would need to exchange USERNAME with the real username in this little bash script. This should work, use at your own risk. (^.~)
#!/bin/bash
doveadm mailbox list -u USERNAME | while read -r i
do
doveadm expunge -u USERNAME mailbox $i DELETED
done
Finding the real issue would be best of course.
Martin
