"Unknonwn Artist" only on Google Music with corred ID3 tags
Bit late, but maybe I can help someone else who finds this.

I also had this apparent problem and in my case it seemed to be related to album art. I managed to fix the relevant files by re-saving the album art in GIMP (without changing it) and embedding it back into the MP3 file. I had to do this for four files in my library (all of which used jpeg for the album art).

Files with these particular images embedded in them seemed to completely break ID3v2 parsing. Could also be a file size thing, more testing required. Here is one image that gave me some trouble.

The music apps could however, read an ID3v1 tag if it was present, regardless of the image.

This was a problem across the built in Xperia music app and Google Play Music (I'm guessing they use the Android API to enumerate music?), however Poweramp and Rocket Player didn't seem to care, nor did anything on my desktop.

Hope this is of help to someone.

Cheers.