Mailbox folder will not maintain 'Junk' status

I have two accounts each with a Junk folder. The Mailbox Type is set to Junk which allows me to right click and choose Empty Folder... Possibly related, I use Spam Sieve and have a Train Good and Train Spam folder in each imap account. The training rules work well, but every day or few, the junk mailbox loses its Junk status. I discover this because the ability to empty the folder grays out and becomes unavailable until I reset the folder property to Junk. I have noted this behavior over several months, but it is not reproducible nor predictable.

What could be causing the property not to persist?

Mark