Apologies for a long delayed response - I just started looking into this again, and it is still happening with 3.1 RC2. What I noticed now is a line that appears in the error log:
8/30/2017 8:37:38 PM - File exists. [C:\users\teb\Temp\01 - Return to the Moon~000007.mp3]
Sure enough, while the file in the specified save location has no tags, the file in the Temp directory has the expected tags saved correctly.
And to answer your question, I am checking by inspecting the tags directly via both MusicBee and Mp3tag.
Thanks!