My thoughts are you’re possibly hitting some kind of system limitation - see this link, albeit it applies to locally held library playlists…

https://support.sonos.com/en-us/article/imported-playlists-missing-from-the-sonos-music-library

To get a definitive answer for the YTM service playlist, or track, limitations, you are perhaps best to speak/chat to Sonos Support via this LINK.

Sonos recommend music services limit collections to 500 items as more than that can hit timeouts in the system.

Something happened with YouTube music. I have a playlist that has 545 songs. About 2 months ago it was showing the full playlist. All of a sudden it would only show up to 495. I called support and they said they know about the issue. The guy said it might be solved by an app update. The last update on the play store shows jun 13th. So it's been about a month since I've gotten an update. 

I have a similar issue. I also use YouTube Music. About a month or two ago the Sonos app only started showing a small portion of my playlists of only 197 & 249 songs respectively. Under half the 500 recommended max. (if that is true). It also happens on the PC version of the app.

After selecting the playlist, I can access the queue and it shows the complete playlist in the queue. This is a pain though because if I want to play a specific song in the playlist that is not appearing, I first have to start playing the playlist, then go into the now playing queue to select the song.

Come on Sonos, can you please fix (I even have shares in your company  )