Interesting! I wonder if it's a Wayland on KDE-specific thing. I'd read it as a security and efficiency feature of Wayland's, where it suspends applications that are not in the viewport. An example case is that I'm playing FFXIV on virtual desktop 1. On virtual desktop 2, I have whatever thing I'm looking up or collecting in the browser. While I'm on virtual desktop 2, it suspends the game on desktop 1, which wouldn't be a big issue in single player games as much, but in games that call to a server, like FFXIV or Overwatch, it disconnects you from the server.
I was able to get around this by using Windowed Fullscreen and turning on Caffeine so that my computer wouldn't sleep, but the turn screen off feature also changes the viewport, so I've had to disable that feature entirely.
Then I work from home, my home computer is hooked up to the sound system, so I'll play music through it, but then be on my work computer next to it. I'd like my computer to still black the screen and lock, but the moment it does either, viewport changes, music cuts off.
If it's viewed as a security feature, then the fix should not to have to be preventing your screen from going off or locking. I'd loooove the ability to set certain apps from suspending when not in the viewport. Bonus points if it's one of the features added to gamemoderun