It's not clear if the way it worked before was the way Apple intended or not. Usually the app in the foreground controls displays, which wasn't a problem on iPads until the definition of foreground changed. It could be that an app was /supposed/ to let go of external displays when leaving foreground, and not it's just enforced. Unfortunately, Apple gets to decide what is the correct behavior, not the user.
> Unfortunately, Apple gets to decide what is the correct behavior, not the user.
Apple used to put a lot of money into user experience to get the behavior correct for the users. Presentations are an important niche that that group wouldn't have ignored. What is correct presentations is probably wrong for everything else - but you still need to make that group work correctly since it is important.
I'm not sure what happened to user experience groups everywhere. They seem to have lost and the world is worse for it. Though at least they did set some good patterns that remain to this day - but only because the people in charge haven't changed it.