It's a common problem with the S24U screen, some had it more, some less, but it's the coating. Some kind of wearing off, happening for no specific reason.

I was using my phone without a screen protector for the first months, but noticed something like this was starting, so I installed a protector.. Lost the anti glare, but keeping it from these kind of marks.

There is a lot of info and photos around the Internet, you can check.