I had an album not showing up in the library (but it played when dragged and dropped in the play queue from explorer) and found the reason:
It contains a song called “4°”. I deleted the “°” out of the file name and the album showed up immediatly in the librabry.
So the metadata is not the problem, but the character “°” in the file name.
OS is Windows 10 with NTFS and files are on an internal SSD.
No big deal, but maybe this can be fixed in the future