I always assumed those were the result of my cycling backpack putting too much pressure on the screen. It's basically bow-like structure to keep the bag off my back.
I do have my MacBooks cooking themselves in my backpack every so often too.
Basically, every MacBook's I had in the past > 10 years ended with that issue.
I can confirm that this is the issue. I have little white spots on the display, and they appear during my motorcycle trips, when my luggage is pushing against the display. I got a fresh one from the latest trip. Armoured cases are not nearly as armoured as they should be.
I do have my MacBooks cooking themselves in my backpack every so often too.
Basically, every MacBook's I had in the past > 10 years ended with that issue.
Mystery solved!