I suspect that the Oppo is using Thumbnails rather than Cover Art images. Not sure. If so, this is normal. The Thumbnails database is specific to each MC installation.
MC needs to rebuild Thumbnails after each major version upgrade. You can run that manually to speed up the automatic process.
Options > Tree & View > Thumbnails > Build missing thumbnails, for settings.
Theoretically, you can copy the old Thumbnail database over to MC23, but I wouldn't recommend it, particularly if the upgrade has been in place for a while, as MC would have started building the Thumbnails. You will find the Thumbnail databases in "C:\Users\[User ID]\AppData\Roaming\J River\Media Center xx\Thumbnails\" where "xx" is the MC major version number.
If that doesn't fix it, then the Oppo is probably using Cover Art images, and expecting them to be in a tag the audio file. Do you have the Cover Art images embedded in the file as well as in the folder? Check to see if the Cover Art is still embedded in the file. See:
https://yabb.jriver.com/interact/index.php/topic,107951.msg771645.html#msg771645 That thread describes a problem a few people are having with embedded Cover Art.