I tried to reproduce this and the only suggestion I have is that occasionally the "send selected to order" button at the top gets changed to another provider. Have you tried looking at other message boxes to see if the message has gone to someone else (maybe a still active user who is no longer at the practice)?
If this is not the case, I would suggest that this is an AC bug. I would talk to support and show them some examples.
Of course if you can determine some pattern (only with orders in certain categories or going to a particular provider) then that would be helpful.
