There would be a .thumbnails folder in the Music folder which contains compressed pic.jpg in which Google play music will then treat it as the album cover of folder X1.
None of the above actions ever required me to edit any tags or anything.
I wonder why does it not work anymore.
Any ideas in how I could trigger these functions? I tried many things, clear data Google play music, clear data MediaStorage, clear .thumbnails, nothing seems to work.