Thank you for looking into this, Florian.
Let me give you more details.
I am recording from vinyl using a program called LPRecorder v8.0.1 from cfbsoftware on XP. It is a very old program which I will be upgrading when my laptop dies. It converts to wav. I then use the companion program LPRipper v8.0.1 which breaks the wav into tracks. It then converts them to wma. I then have to copy to thumb drive and transfer to my main desktop. They are not locked or protected; no other programs that could be accessing the files are open (file mgrs or media players).
The problem is inconsistent. It seems to only happen with virgin ripped files. When I open a directory with previously ripped and tagged files (no doubt tagged with an earlier version of Mp3tag), the problem does not exist, but that's hard to verify.
I suspect it is caused by something that the old version of LPRipper does to the tags. I can send you screenshots that will illustrate what is happening. I can also send one of the problematic files after ripping and before and after tagging.
Meanwhile, I will download a trial copy of the latest LPRipper, and see if that solves the problem.