Here's the likely answer as to why it's not working. I use split-screen as much as you do in the same manner (ie. Youtube always on top and change the lower app by hitting the home key and selecting an app, opening the app drawer, or use the switch apps button at the bottom of the screen). Android/Samsung must have felt that a lot of people weren't using their precious edge panel so now they are kind of forcing you to.
The work-around (I hate this term) is that you access split-screen like normal. Then, hit the center three dots and select the save icon and this "view" will be saved to the edge panel. The problem is that every time you want a different view you must save that view to the edge panel and if you're like me and use split-screen often, you'll have a plethora of saved views in the edge panel to navigate through to find the right one. Also, I hate the edge panel as it somehow always finds a way to be accidentally accessed.
(Galaxy Note 10+ 5G)