While I have seen a few reports about this here on the Apple forums, I have yet to see any of our organization's Apple Silicon Macs have this issue yet. A couple users posted pictures here on the forums, but the pictures have never provided enough detail to determine the issue. It is something that must be seen in person...even then it may be difficult to determine the cause whether it is a materials defect or something external is causing it. I still don't have an opinion one way or the other yet.
So far I've only seen only a few reports here on these forums so it is not a wide spread issue at this time. If it were a wide spread common issue, these forums would be full of posts & threads with complaints.
Make sure not to let any chemicals touch the screen since some chemicals can damage the screen. Also, make sure the spots are actually pits and not just a bit of dirt which is tough to remove. There have been a lot of times over the years I have encountered displays which a bunch of "dots" which turned out to be some sort of residue from something being splattered. It has been difficult to clean off even with an LCD screen cleaner.....some times I've had to gently use my finger nail to rub off the spot.
You can always provide Apple with product feedback (Apple won't respond):
Product Feedback - Apple
You can also contact Apple corporate (Apple may respond):
Contact - How to Contact Us - Apple
If it occurs again and Apple won't replace it, then you can check out your local consumer protection laws to see if any of them may help you. Usually it will need to be shown it is indeed a material defect for Apple to be required to cover it. Obviously you were lucky that the Apple tech was willing to consider a defect the first time, but since it has not been reported as a wide spread issue, it is understandable a tech would be hesitant to offer a free replacement a second time for the same laptop. I know I would be hesitant to do so as well.
Hopefully it won't occur again.