[MlMt] Can't change message status to UNread

[Randall Meadows]

Is this a known (or a new) bug, or in the midst of my other MM woes, 
have I screwed up some setting?

I am utterly unable to change a message's read status back to unread, 
for ***most*** messages, although I haven't been able to figure out 
exactly what the trigger for it failing is.  Upon an attempt to mark a 
single read message UNread, MailMate immediately reverts it back to 
read.  If multiple messages are selected, this does not happen.  You can 
see this in action in the following video screen capture:

https://www.dropbox.com/s/s26ico2v936zo8i/MM-Cannot-Unread-1-Message.mp4?dl=0

This becomes particularly problematic when the message I want to change 
is the ONLY message displayed!
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