I've had this issue since the new client came out (a couple months ago now, I think). I've got a Pixel 2 Android phone.
The old client used to keep the phone awake, even if you didn't touch the screen. The new client does not.
I've got a very short time on my phone before it dims the screen and goes to sleep, in order to preserve battery charge (which I need to do because playing DK tends to eat my battery...). If the screen goes black while DK is up, I am immediately disconnected. The result is that if I am inattentive (for instance, if I am playing an easy event raid...or, occasionally, just an easy PvP raid), the screen will go black and, even if I immediately turn it back on again, I'll get disconnected and lose my raid.