Have you seen this on any other type of file (e.g. a plain text file)?
One idea that springs to mind is your virus checker is detecting the change in the file, opening it to check it, which is blocking Opus from reading the new metadata from it.
I would have thought pressing F5 would be enough to refresh the label - needing a full program restart is odd.
Yes. It happens on plain text files too. It's perhaps also worth noting that once it has happened to one file, attempts to label other files also (apparently) fail. After restarting Opus they all show up correctly.
That suggests the background thread that reads the metadata has got stuck on something.
Can you use Task Manager to make a dump of the dopus.exe process when it's in that state, zip the file and send it to crashdumps@gpsoft.com.au (or via dropbox etc)?