The inconsistencies you're experiencing with the cover screen media player on One UI 5.1.1 may be due to a combination of factors, including both the app developers and Samsung's software. It's possible that these issues will be addressed in the final release of One UI 6, but it ultimately depends on the updates and improvements made by both Samsung and the app developers. Keep an eye on future software updates for potential fixes.
Thats with Samsung's native music player, and is a huge downgrade from previous models. It seems like music playing in general wasn't optimized very well. All these apps worked fine on my previous flip.
I really hope it gets fixed in the next update as well.