A smudge on my IPS LCD monitor on grey backgrounds and more noticeable at low refresh rates, what is causing this?

I’m having this issue on my ASUS PG279 monitor where on grey backgrounds I get this smudge on my display that is darker than the rest. The lower I put my hz the more noticeable it becomes, and it becomes noticeable even on white backgrounds, I’m wondering what this is, maybe it has something to do with refreshing. I have even recorded a super slow-motion video showing the issue in action. https://youtu.be/zb56kDumYQ4. Video was when monitor was set to 24hz.