I think this might not be that rare. I have owned three Thunderbolt 3 15" MacBook Pros. One was the 2016 model, and two were the 2018 model. All three of them developed these bluish spots. Two of these three were provided by my employer. I noticed a coworker with the same blue spots on a 2017 15" Thunderbolt 3 MBP. After my experience with them on the 2016 model, I was much more careful with the 2018 models. I carry them in a laptop backpack when I travel and I also use a neoprene sleeve for extra protection for my personal laptop. I rarely use them in clamshell mode. My personal one, in fact, never gets used that way. I do not use alcohol wipes on the screens. My work laptop had a 3M screen protector on it for its entire life, and so it could not have absorbed oil from my skin. Despite all of these precautions, both laptops developed the blue spotting.
I do not believe that this problem is due to heat or chemicals on the screen. One possibility is that it may be due to pressure on the screen. My laptop bag is quite heavy. However, I would have expected the neoprene sleeve to help with that. Also, it doesn't explain why my coworker has also seen this problem.
Thus, I think the most likely explanation is that this is a manufacturing defect that worsens over time and not something that is caused by how it's used.
Edit: I forgot to mention my coworker never noticed the blue spots until I pointed them out. That is one reason I think this might be more common than people realize. Unless you're looking for it, many people seem not to notice.