I found a work around from a 2019 forum post I came across. I selected all the songs in my library clicked on "song info" while they were all highlighted. In the info most everything said "mixed" because it was all the songs. Under "comments" I typed a "k" so it overwrote all the comments for every song. That took a bit of time. Next day, I hooked my iPhone up. I went to the section where I added "selected songs/playlists/artists/etc." instead of the entire music library. After choosing all the artist and albums I wanted, I hit "Apply". It went through backing up, diagnostic stuff, etc. and basically did nothing. When it was done, I hit the "Sync", previously the "Apply" button, and now it's finally copying the library I selected.
This is a work around. There has got to be some kind of problem in the code/software because it used to just download with no problem. Only thing I did recently was update my phone to the most current operating system. I will try a couple of other devices, like an old iPod and another phone that's not as currently updated and see if it is a problem for everything.
Hope this helps with you.
