"Unknonwn Artist" only on Google Music with corred ID3 tags So, before you dismiss this question please continue reading for all the details.
I have my music collection on an SD card on my phone. All my music is very well organized, I'm almost OCD about it. All my music is properly named: AlbumArtist/Album/track# - title.mp3
All my files are mp3s, all of them are using only ID3v2.4.0 with no ID3v1 (some people use both [itunes?] ID3v2.x and ID3v1, not me). Every single tag field is using UTF8 encoding. All my music has an Artist and an Album Artist field, genre, track number, album art (properly encoded), date, and album, strictly no other field. None of my file-names contain special characters either.
I've gone even to the extreme of creating scripts on my computer to make sure that every single mp3 file on my collection is stored with the previous parameters.

Now, there are a couple of songs the Google Music keeps showing as "Unknown Artist", no track-number, no album art, etc. even after all the previous efforts. Other music apps (i.e. Rocket Player) show the correct info without a problem.
I've clear Google Music data, reinstall it, reboot phone, but nothing fixes this issue.

I've searched online and this forum. There are two other questions related: 


&#39;Unknown Artist&#39; in Music Player.
Google Music &#39;Unknown Artist&#39; only in Android App


The first is totally the opposite of me, where I'm very knowledgable of the ID3 tag standard, how to edit it (I've created Python scripts to automate the process) its version, encoding, and fields. 
The second is related to FLAC and none of my files use any other format besides MP3 (with ID3v2.4.0 and UTF8)

Does anybody have any idea of what's going on? Why doesn't Google Music want to read the ID3 from the files? It feels like it's getting its info from an old database and not the files themselves.

PS: I'm using Android 7.0 and latest version of Google Music at the time of writing (v7.10.5022-1.T4178353)
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.