I had Mate 20 Pro with LG display and got it exchanged due to display issues (greenish in total darkness, low brightness). Now I got with BOE display, but I found out one issue that I had on LG screen too. Check the video that I recorded back on LG display. Exact same issue is on new phone with BOE display. To explain:
watching YouTube video in full screen and after a minute or two I exit the full screen and want to check notifications. When I slide down notification bar, the white part of YouTube with description of video is part black, part white.
Same goes if I play for example fortnite. Loading screen show like it is burned. I have new device for less then week.
Any idea, is this display again or something else? No, it's not normal for OLED displays, at least not by my experience in all years with OLED displays..