But reading the tags of both should give you some more information.
Thanks for the quick response and suggestion, which indeed provides more information but has not led me to an answer.
(1) Binary comparison of the old and new flac files shows they are byte-for-byte identical, with the exception of two bytes. In every case the TOOL VERSION changed from 30.0.
19 to 30.0.
30. As expected in this case, comparison of the individual tags written to the files shows them to be identical except for TOOL VERSION.
(2) Looking at the other library fields in the MC tags panel, old v.s. new all appear to be identical AFAICT, except for DATE MODIFIED which has been updated as expected between the old and new versions.
So the question remains: What is the root cause of the update?
A possibility, but I do not think it holds water: I have one custom field in MC library manager, called TITLE, with Edit Type Standard, which I recently changed to Calculated Data with Expression =[NAME]. I suppose it is possible MC "updated" this field in the background without actually changing its value. But if this were the root cause, then I am puzzled why the great majority of my files were not affected.
The changed files all had one of two Last Modified dates back in mid October, 2022. These files encompass a wide range of genres and original import dates, most were not recently played, and I do not use star ratings. My music folder hierarchy is organized in common fashion by Artist > Album, residing on a single USB SSD attached to the MC PC. Under a given Artist directory, typically some but not all albums were affected. In every case, all tracks within an affected album were changed in the same way with same update time stamp.