That might be the backlight flickering, PWM is used and there are some reports that later purchased models now have display panels that flicker at a lower frequency, thus making it more noticeable. It is probably that, because it usually intensifies at a lower brightness. Essentially all current 13 and 14 series iPhones have a chance to have this issue. You might be able to fix it by returning the phone and buying it again, especially if it is older stock. But you might not be able to get a non-flickering one.
Depending on how sensitive you are to PWM flickering, you might have to read reviews before purchasing something and making sure the PWM frequency is as high as possible so you can't notice it. Sensitive people can notice it with some devices even at 100% brightness.
If possible, I'd bring it into an Apple Store within the 2 week return time and ask for a replacement on the spot, set it up right then and there, and if it immediately shows the same issue you can then do a return in the store as well. Personally I never noticed such a flickering even on the worst of displays, so I have no idea how bad it is, but many people report this issue especially with iPhones, so Apple store employees should be aware of it.