VIZIO P55-F1 TV with backlight issues

Several months back my TV was hit with a surge after the only ground to my house was disconnected (thanks, landlord). Even with surge protection, some electronics got hit but thankfully most of my major devices were fine. My 55" 4K TV seemed ok at the time but I have since experienced issues with it sporadically turning off or backlight zones going dark.

I hired a TV repair guy to take a look and he claimed that the LEDs are not damaged and the needed fix was to resolder all points on the boards. He did so, TV seemed to work ok in his shop (he left the CNN app running for the better part of the day with no observed issues), and it worked ok in my house for about an hour after he brought it back. Previously observed issues have since returned, however.

This is my attempt to crowdsource advice/tips, since the repair guy is struggling to find a fix. It is back in his shop and he's now talking about potentially replacing the main board, which seems suspect to me (if anything, I think the power board should be tested/replaced first).

TV model: VIZIO P55-F1 55" 4K TV

1. Backlight is on but some zones are dimmed/off. It's usually the same areas but sometimes other areas are also dimmed. This happens frequently, usually a few minutes after powering the TV on. Restarting the TV sometimes fixes the issue temporarily and sometimes doesn't.
   1. Most common pattern: https://photos.app.goo.gl/xEg2oazMNHbXmp...

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1. Example of another pattern observed: https://photos.app.goo.gl/Eze9BCDnsYTKM2...

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1. Backlight is not on at all. This has happened at least twice, normally after powering the TV down then back on.
   1. In this video, you can see I'm adjusting the backlight setting but the brightness of the display never changes (showing the backlight is not functioning): https://photos.app.goo.gl/AHFK8iiwWriKHq...
2. Backlight flickers at high speed. This was observed twice in the course of my cycling the power on and off to fix Issue 1.
   1. Video: https://photos.app.goo.gl/bPw8x5gcmsAGXe...
3. TV turns off at random. Normally 1-2 hours into the TV being powered on, it will simply turn off. This is the longest-running issue since the power surge event, even before backlight dimming issues. I have not observed it since the TV was returned to me (after repair guy performed resolderings) but admittedly did not use the TV very long before other issues were observed and the TV was returned to the repair guy (so it may still be an issue).

• The issues were observed regardless of media input type. I tested using a PlayStation 4 Pro, Xbox One, and the native smart apps on the TV. The same dimming/backlight issues occurred on all of them and switching input type did not fix the issues.
• I turned off all TV features that automatically adjust backlight levels in response to the image displayed (Auto Brightness Control, Black Detail, Active Full Array, Clear Action, Film Mode). These features might otherwise account for certain zones being dimmed but because the features were turned off, we can rule out that they are responsible for the phenomena.
• For testing, I use a feature on my TV that provides a solid white image. This most prominently shows the issue where certain zones of backlight are dimmed/off. The feature can be found by going to Menu (button) > Picture > Color Calibration > Calibration Tests. From here, select Flat Test Pattern and set it to 100%. Here's a video of me setting it up: https://photos.app.goo.gl/TioHrtfF1QaXVd...