The YouTube Music app blocks you from navigating to a different song at the same time as playing a track if it decides that the track is primarily aimed at under 18s (such as the theme from a retro cartoon).
It's UI might charitably be described as a total catastrophe.
Well compare that to Spotify which can only show videos full screen in one orientation and needs to be force stopped constantly because podcasts block a lot.
Also, I never encountered the problem you complain about.
The YouTube Music app blocks you from navigating to a different song at the same time as playing a track if it decides that the track is primarily aimed at under 18s (such as the theme from a retro cartoon).
It's UI might charitably be described as a total catastrophe.