Last update:
I downgraded to .84 and then to .80 with no change (these were the 2 most recent in my downloads folder).
I then looked in the AppData\Roaming\J River folder I backed up before closing J River, and saw two extra "browser (theater view)" files, one called "browser (theater view) (1).jmd" and the other "browser (theater view).jmd.tmp". These had each been last modified about 24 hours ago, with the ".tmp" one being a bit more recent. All of the other files in the backup folder looked properly named when compared to my new AppData\Roaming\J River folder, and there were no other extra files.
I replaced my new "browser (theater view).jmd" with the ".tmp" one and restarted J River, and all is now well - at least, I don't see anything wrong/missing from what I can recall.
If you want to look into this, let me know if there are any files or bits of info I can provide. I have disabled automatic updates for now - possibly for always, with a manual backup just before doing each update in future.