Flac metadata not read properly

Good to see more people are now starting to complain about this and not accepting archaic metadata handling from 10 years ago...  fix up Pioneer!

c.

But actually you can see both tags if it's a mp3

Yes, you can see them, but RB does not respect the de-facto field delimiter convention for id3 (the mp3 container/metadata format...). 

So in terms of what you see, if you have a multi value genre definition:

In an MP3, let's say you have:

Hip Hop; East-Coast; Instrumental

(or technically correct:

Hip Hop/0East-Coast/0Instrumental)

What RB does is display the above as a single genre, with the name:

Hip Hop; East-Coast; Instrumental

Whereas if this would have been written correctly, as per id3's multi value de-facto convention (as this only became official in id3 2.4 only officially, but everyone and their mom is using this de-facto standard where.   see:

http://id3.org/id3v2.3.0   - composer separation)

The above SHOULD be displayed under EACH genre, i.e.

Hip Hop

East-Coast

Instrumental

So NO. RB does not handle multi values corretly in mp3s either.