This is most likely an LCD/display problem, not a sensor issue. If the camera sensor were at fault, you’d usually see the defect in the recorded image as well, not just on the rear screen.

Spots and lines that grow over time often point to physical damage or failure inside the LCD panel itself, such as pressure damage, liquid-crystal leakage, or internal layer failure. That can cause discolored patches, dead areas, or lines to spread.

A practical way to confirm it: review footage on a computer or external monitor. If the files look normal, the sensor is probably fine and the rear LCD is the problem.

In short: the issue is best described as LCD damage/failure, and the growth suggests the screen itself is deteriorating rather than the imaging sensor failing.