Screen retention would have specific marks that matched whatever is displayed for a length of time, that image doesn't resemble anything I've seen on OLED TV's. So highly unlikely in 3 weeks and I'd say it was faulty. Could be finger pressure marks?
Thank you for your help.
I've solved the mystery. The burn in marks were captured onto a screen protector I was using.
When I removed it, ready to send it away to be repair, the screen below was perfect.
I have been using a wireless charger, which has rubber "cushions" and the position of the seems to match the marks on the screen protector.
My guess is something on the phone got really hot when charging and left the marks on the screen protector.
The screen protector's surface was clean. It's just strange.