Tracks appearing in ‘groups’ they don’t belong to

[mv70]

Synology/bubble upnp/naim kit/naim&kazoo&lumin software.
I use mix of wav & flac. I edit tags with kid3 on a Mac.
I’m at latest version of all software on all platforms.
I use my own named tag to represent my favourite tracks - 5 is high 1 is low. In minimserver preferences I specify the relevant parameters. These render fine in all the above software and they appear in a group which I call rating. Selecting that group shows 5 subgroups; labelled 5,4,3,2, 1. Within each sub-group tracks are listed if they have been tagged with that sub-group’s value.
For 2 years this has worked without issue. The other day I found tracks in the sub-group 5 that are not tagged at all, so they should only appear in the default sub-group called unknown. The same is true for the other subgroups. What it is odd is that a who;e album of tracks without any track tagged as a ‘5’ appeared in the sub-group ‘5’. There weren’t many, but they seemed to be random. When I edit the offending tracks in kid3 it shows they are not tagged to be in the group.; just checking if I had not changed by mistake. By changing any other tag, saving it in kid3 and then rescanning in minimserver, the track disappears from the sub-group. It’s not the rendering software as all three of the above show exactly the same offending tracks. So I am working my way backwards. Any help welcome thank you.