To conclude this I took the Fraunhofer IIS 7.1 test track, these are usually provided in AAC but I converted to LPCM to test. This plays back correctly. I then converted that LPCM to FLAC using xld and that also plays back correctly on LibreElec 12.0.2 / kodi 21.2
The FLACs from the github repo playback correctly using macOS's built in support, which does something funky with virtual surround with the multichannel files. However from my testing do not playback correctly on LibreElec 12.0.2 / kodi 21.2. However from your testing mostly play back correct on LE13 nightlies however I'd rather not update my setup to a nightly to test this.
My aim is to play 5.1 FLACs of a variety of choral music which was originally released on SACD but Audite have now released as multichannel FLAC which can be bought from their website.
I guess even if the bought FLAC ends up not playing correctly I can convert it to LPCM and that should work.