This appears related to a maildrop package update pushed out in the last day or two. I was able to downgrade it using this command:
aptitude install maildrop=2.0.4-3
Then do this:
And now my queues are flushing fine. I am also using the following command to prevent this from re-updating, which I will 'unhold' at some point in the future when I see the issue has been resolved...
aptitude hold maildrop
Good luck folks, I had a lot of mail getting queued but seems OK now.