Stuck pixel on iphone xs

Today I noticed a very fine white horizontal strip on my display. I immediately scheduled an appointment at an Apple store.

The problem is that one can only see the strip in dark surroundings. So the apple staff couldn‘t see the strip and told me that nothing is wrong with my display. They told me that you would see dead / stuck pixels even in light surroundings.

Now my question: what‘s wrong with my display? What would you recommend to do? I attached a photo of the white strip. Pls ignore the grey strips!