My 10 month old FP5 running stock OS is regularly used in bright sunshine outside (I am an Ingress Player and play sometimes for several hours a day). The status symbols at the top line are permanent burnt in and visible on bright backgrounds. There disappers nothing after a few seconds.
I also have some mild burnin. The clock on the top left, wifi data battery and battery % are all faintly burned in on the top right of the screen. And the gesture pill on the bottom.
Anywhere else, there isn’t the smallest sign of burnin.
What’s concerning, is that I’ve noticed this about six months ago. And the phone is about to turn a year old.