Hello,
I have a Shanling M3X and similar problems. Some of my music lack tags. In most cases I see Unknown Artist, but the rest of tags are ok.
However, I have one album where random songs lack both artist and album tags and no album art. No such issues occur in other music player using both Android and Windows.
My library is almost 100% flac files, mostly 16/44, some albums 24/44 or 24/48.