Apple Watch internal storage decreases with iCloud Music Library enabled

I recently noticed that the Music app on my Apple Watch (Series 2, watchOS 4.3.1) takes up HUGE amounts of storage space, i.e. up to 2.5 GB out of the available 5.5 GB, although no music is synced to the phone; all playlists are disabled for sync in the Music app. Furthermore, both, the Music app itself on the Watch as well as the 'About' section under 'General' indicate zero songs.

Further research led me to the reason for the storage use: as soon as I enable iCloud Music Library on my paired iPhone (iPhone X, iOS 11.4), the internal storage of my paired Apple Watch starts decreasing. When I check the ‘Usage’ via the Watch app (General -> Usage), I can see that the Music app on the Watch is responsible and uses huge amounts of room, up to 2.5 GB (out of its 5.5 GB) within a few days after turning on iCloud Music Library on the iPhone (it is increasing slowly but surely over time). When I check the Music app itself on the Watch, there is still no music synced.

When I turn off iCloud Music Library on the iPhone (Settings -> Music), storage space on the Watch is quickly increasing again and within a few hours, the Music app only takes roughly 200 MB of storage on the Watch (as can be checked through the Usage setting on the Watch).

This is reproducible, i.e. when I turn on iCloud Music Library again on the iPhone, storage space on the Watch starts decreasing and when I turn it off, storage space is regained.

What exactly is this storage on the Watch being used for? There is clearly no music on the Watch; when I am out of range of my iPhone, I can't play anything. Is this a bug? I can't imagine that this is a feature… 😕

Does anyone have any ideas?? Any help is appreciated!

P.S. I have already tried to unpair and re-pair the Watch to the iPhone. I also already tried to hard reset the Watch and start from scratch (set up as new watch, not from backup). Finally, I paired the Watch to a different iPhone. In all cases, the issue starts to appear as soon as I enable iCloud Music Library on the paired iPhone...