Smart playlists not syncing between Music and iPhone

I noticed recently that one of my smart playlists (that I've had for ages) was much shorter on my iPhone. Upon inspection I realised that it now contains only 14 songs (instead of the 90 that the same playlist contains in Music app on my Mac).

Wondered if it was because the songs had somehow been removed from my phone, but no, they're still there - just not included into playlist for some unknown reason.

I've since realised that this has happened to several playlists. They are fine on the Mac, but are not syncing on the iPhone app. Weirdly this has happened (I think) between syncs, not as a result of a sync error - so I suspect iOS / app update issues.

I tried removing the playlist from being synced.. then syncing, then re-adding - no luck. Same problem.

With one playlist I tried crating a non-smart playlist of the same songs and that works - but that's isn't a workaround... as the smart automation is the value of it.

Anyone had this or know how to fix it?