Sony XBR-49X700D: Vertical Lines

[lothian]

Sony XBR-49X700D
Manuf: 2017

Yet another post complaining about vertical lines in an LED TV...

I have a Sony XBR-49X700D that decided one day to display a black screen and the dreaded "Sony Six Red Blinks". I buried my face in my hands, fully aware that the "Sony Six Flash" error code typically means either of two things: a failed T-CON board, or--more likely--one or more dead backlight LEDs. Bleh.

So I did the easiest thing first: I swapped the T-CON board with no change in status. I heaved a plaintive sigh, and began the onerous chore of dismantling the TV to get at the LED backlight. Sure enough, one of the LEDs in one of the boards tested dead. I replaced the bad LED strip with new. The backlight array now works perfectly and there is no red blink code.

However...

Now the display has vertical lines on both sides and a large swath of grey cuts along the middle (see image). Vertical lines typically are caused by a bad T-CON board and/or a poor ribbon cable connection. I swapped between two T-CON boards and re-seated the two ribbon cables that connect the T-CON to the panel--no change.

Surely there's some simple thing I've overlooked. I am stumped. I can't imagine a formerly working panel is now broken to this extent.
What causes vertical lines? What else should I check?