Fluid ingress.
Did someone wipe the screen with a wet cloth to clean it ? I have seen this where the water or cleaning fluid pools at the bottom of the LCD panel and finds it’s way into the backlight to glass panel interface and rises through capillary action. The fluid then acts like a light pipe from the backlight to the LCD panel and appears brighter than the rest of the backlight.
Sadly this is not normally considered a viable repair proposition so the LCD panel is changed.
One trick that can sometimes help, but rarely completely removes the effect, is to get some silica gel desiccant packets…. 250g or larger, dry them in the oven then seal the screen with the silica gel in an airtight bag. Then leave the unit somewhere warm for a few days. The desiccant can absorb any moisture coming back out of the LCD panel and creates a very low humidity environment within the bag. To improve the drying effect it is sometimes helpful to warm the screen with a hair dryer on ‘warm’ setting to promote water evaporation and then place the screen in the sealed bag with desiccant. Do not overheat the LCD panel. Warm is fine, hot is not.
Do not try to use Rice….. it is not a true desiccant and frankly a waste of time.