Longshot, but take a look at the jmd files in this and other nearby folders:
C:\Users\[username]\AppData\Roaming\J River\Media Center 29\Library
Your description sounds like occasionally the appropriate file can't be opened or read. Antivirus is always my hunch with problems related to opening files.
My testing has been done on both PC's mentioned in my Forum Profile plus on my older Win 10 PC, it's too old to be upgraded to Win 11.
The chances of 3 different operating systems, each one with its own AntiVirus and Firewall Software, misbehaving in exactly the same way in only one area of MC have to be somewhere between Extremely Low and Impossible.
If you look very carefully at the screenies attached to my earlier posts, or the 2nd video in the more recent one - most of the tags where the misbehaviour is observed the Tag Window was displaying the tag's content, before the Tag was opened to allow me to edit it. Plus, when MC reverts back to behaving it continues to display the content of the relevant tag.
In addition, over the last 3 months I have on all 3 pc's been making sure that all of my tracks contain all the Metadata I want them to have/display which has involved me in updating lots of tags, including the ones that I've used to help demonstrate the misbehaviour, without encountering any problems.