It appears that the most recent version of iTunes 12.3.3 contains a database error that affects a small number of users, and can potentially wipe out their music collection after the update.
I've given up being surprised about bugs in Apple software. I now subconsciously expect it to not work properly. Which is a shame.
The last sentence says it perfectly:
Now is the time for the company to recognize that without stable base applications, Apple Music will always be plagued with accusations and problems — even if it doesn't always rightly deserve them.