Adding more than 542 tracks in one batch to Your Music breaks synchronisation. Any changes after such an action are not saved.
Attempt to add a large number (542+) tracks to Your Music in one go. Once you trigger this bug, any changes you make to Your Music after will not be saved.
Workaround (to import Starred playlist - provided by @Rollo_ ):
I eventually found the number. 542 tracks will drag and generate Album and Artist entries. 543 did not generate, but DID generate after a restart. Once, but not a second time. Buggy indeed.
So, if you really want to get them all in, do this.
Copy all of your Starred tracks into a new temporary playlist.
To keep it easy to navigate, sort your Starred list fist, by title, for example, so you can remember where you left off.
Open that new one and count the number of tracks on a full screen, and go down as many screens as it takes to get to around the 540 number.
Drag them over, look at the number as you drag,
check to see that they've generated Album and Artist entries, and if they have, go back and delete those tracks from the temp playlist.
Repeat as required.
This bug is pretty annoying, it is worth continually checking your content is being saved when importing lots of content into Your Music by looking at the Album/Artist entries or restarting.
2014-04-26: Reported on 769585 this bug also effects removing tracks from Your Music.
If you are having this issue, can you please post: