When in a full screen game on the primary monitor and I need to pause and do something else, I will Alt+Tab which should minimize the game and set the focus to whatever the next program is in the alt+tab list. That Alt+Tab action should not be making apps move from a secondary screen to the primary, but thats what happens after the a windows 21H1 update.
The only thing that keeps the RDP window on the second monitor is to take it out of full screen.