The owner indicated the flashlights were flashing which typically is a sign of failing backlights. I had asked them to turn down the backlight level and it was fine for a few weeks before the picture went dim. Testing the strip, 5 out of 68 LEDs were bad so I replaced the single strip. Powering up the TV, they would flash on power up before shutting off though they were all lighting up. The strips are wired as 4 banks of 17 LEDs and each bank tested good and measured 51V on my LED tester (3V LEDs). I have run into the issue of bad LEDs damaging the power supply so suspected the controller IC may have been damaged as well.
Searching online, there are a few reports of the OZ9998HDN IC overheating and needing replacement with symptoms of a flashing backlight.
I ordered the ICs from Aliexpress and the power supply did not turn on the backlights with both ICs that I installed. I ended up buying a new power supply board and this finally fixed the TV.
50L3400U is a similar model. I have run into this problem before with Toshibas where there is both LED and power supply damage.