@StreamerUser That makes sense, thanks for the info! But even if the problem is on Apple's side, the issue is happening on the Roku platform and they should deal with Apple to solve it. Not a major thing, but still annoying. Let's hope this gets fixed some day.
Again, AirPlay is entirely coded by Apple, and its behavior is controlled by Apple.
That being said, its more complicated due to the AirPlay apps' OS-level modifications/integration, so there is some enhanced cooperation/communication between the two companies, but Apple's chosen screensaver behavior for the app is going to be very far down (or not even on) the list of "issues" for Roku to bring to Apple's attention - still best to contact Apple directly if you want the AirPlay screensaver behavior on Roku changed.