There are lots of reports on the reddit tvrepair subreddit of NU series TVs with failed backlights. NU is a 2018 model year and backlights are failing after 2 years. These models are edge lit from the bottom so LEDs that have burnt out will cause dark vertical bands or sections.
Some model numbers include
UN50NU6900F UN55NU6900F UN55NU6950F
With the screwless back cover and edge lit design, replacement can be pretty difficult especially on a large screen.
It is always recommended to never run your backlights at maximum which shortens their life.
Another reddit post shows that the LEDs can melt the clear plastic panel that spreads the light behind the panel. When it is melted, the gap will create bright spots on the bottom. The heat can also crack the panel which will show up a vertical bright line above the LEDs.
Here is what edge lit LEDS look like behind the panel. In the NU series design, there are fewer LEDs and they are spaced further apart. They run both brighter and hotter.
Update – I repaired a UN55NU6900F. Details here.
Update 2 – Here is a UN55NU6800F with a burnt light spreading panel and crack caused by the heat from the LEDs. The crack will appear as a bright vertical line in the backlight.