Screen is grey on the edges (Apple Watch Series 3)

I'm using an Apple Watch Series 3 42mm for 19 days.

It works very well, but the edges and the corners of the AMOLED screen seem to be gray-shaded, this is how it looks like:

It can be seen outside, when the light isn't too bright nor dim.

Is it condensation? I washed it once and expelled the water afterwards, but there's no physical damage so the seal should be fine. Could it be that the screen is sinking or wasn't well glued to the glass?

I live in Italy where the temperature's about 14°-18° and the humidity is 78%.

I've already washed the Apple Watch for the second time to see if the problem can get worse, but nothing happened.

There's no burn-in, the AMOLED panel is still good.