difference between flac & wav when using 'Rating'

[mv70]

I use Kid3 on Mac. For flac, wav, mp3 and m4a files I can add the standard 'Rating' tag which displays as 0-5 stars in Kid3. However, this is seemingly interpreted differently accordingly to file type by the time it appears on control point (I typically use Linn Kazoo OR Lumin). I have Bubble UPnP with minimserver on snyology.

I've read the minimserver instructions re use of the 'Rating' tag field, but I'm afraid I don't get it.
Setting the standard Rating tag through Kid3 I get the following results;

1. flac files (vorbis) rated with 5 stars appear in a group called '100', 4 stars as 80 etc. on the Kazoo/Lumin. I found the reference to changing these 'number' groups to for example groups called 5 Stars, 4 Stars etc using the tagupdate parameter. This works fine for flac, but ignores wav & mp3/m4a.

2. Wav files (ID3v2.3) rated with 5 stars (through Kid3) appear in a group called '5', 4 stars in a group called '4' etc on Kazoo/Lumin.

3. For MP3/M4a the standard 'Rating' tag causes the track to be lost from the the rating scheme on the control point.

I was trying to move away from my own custom tag just in case things change in any element of the 'end to end' system, but it seems I need to return back to my custom tag (using the minimserver aliasTag feature) as the only way to achieve a uniform result for rating tracks across all file formats.

Is there a standard way to tag different file formats for track rating and for them to appear in the same rating group in the control point?

thank you.
cheers, marinko