Any EFB with fixed (and bright) UI interface elements will (eventually) cause screen burn in OLED displays.
It this case it was Avare, but it'll happen with others as well.
Had to find the old phone and take some pictures of the damage.
UI layout:
View attachment 155729
Examples of the UI burned into the screen, visible on a bright background (you might need to open them full screen):
Bottom menu bar:
View attachment 155730
Top flight data bar:
View attachment 155731
That damage was done in about 250-300 hours.