I had this problem with lg stylo 4. I was keeping a small flat key on the back of the phone under a protective cover. A white spot half inch diameter appeared in the center. I understood what was happening when some people talked about pressure on the phone. When I opened the cover and removed the key the spot disappeared progressively. Initially when the key was put under the phone's protective cover it stayed there (I don't lock my room) and I didn't see the white spot at the time. It appeared a few days later and I failed to relate its occurrence to the key that was pressing on the device from the back.