Google Play Music not recognizing embedded cover art

For about a year now, any mp3 files I upload into Google Play Music that I have edited with MP3Tag do not show any artwork. I have to go manually add the artwork. It used to work fine. I thought maybe it was something on the Google side, but then I found out mp4 files DO display the embedded cover. I just downloaded tag & rename to do a test, and sure enough, the files edited with T&R showed the cover art in GPM just fine. Is this unique to me? Is there a setting I need to adjust? I don't want to go back to T&R.

The easiest way would be to compare the files that show cover art and those that don't.
Use the extended tags dialogue for that (alt-T)
and pay particular attention to the tag versions that you find, read, write and delete.

You can filter for the property of embedded pictures with
%_covers% MISSING
which shows all files that do not have embedded covers or with
%_covers% PRESENT
for files that have embedded covers.

When I compare the ones that upload the art successfully, the cover type is set to "Other". The ones that do not upload successfully are set to "Front Cover". However, if I cange the type from "Front Cover" to "Other", it just reverts back to "Front Cover" when I save. I cannot seem to save a change of cover type.

I cannot reproduce this. I can change the cover type to any typ I like including "Other".
But as "Front cover" is a valid attribute for embedded covers in MP3 files, I think that this is a bug in the player.
Or your files are somehow up the spout and it would be worthwhile to check them.
See How to check files for errors?

Which tag types have you got in the files? ID3? APE?

Definitely not the files. Literally 1000's of them would have to be bad.
Tags in files say APEv2 (ID3v1 ID3v2.4 APEv2). Is that too complicated?

I just saw another thread saying APE tags can be an issue. I'm gonna mess around with that. I'll let you know what happens.

and another thing: at least Windows has a problem with V2.4 tags. It would be better to use V2.3 tags.

A quick check with the linked utility "mp3val" would have told you that there is garbage at the end of a file which is an indicator for APE tags....

I'm just gonna state the obvious and say I'm too dumb to understand a lot of this, though I am making progress! So in the utility when it says "Wrong number of MPEG data bytes specified in Xing header (5482608 instead of 5480306)" is that the APE garbage?

I really appreciate your help with this.

No, this is an indicator that (perhaps during ripping) the header for the variable bitrate was not written correctly - a slight indicator that not really all files are OK.

You did it. It's working now. For anyone else who comes across this, Google Play Music wants ID3v2.3 tags. I used UTF-16 and cover art is now showing. All credit to ohrenkino!