I just noticed something on WhatsApp. When I play a video on YouTube (I use dark mode on YouTube). And than click on a WhatsApp notification to go to the chat my WhastApp is appearing in dark mode without turning it on. When I go back to all the chats and go back to the chat I was on it dissappear.
So is WhatsApp testing dark mode without knowing? I'm a WhatsApp beta tester and no I don't have any toggle to turn it on or off.