[Philips, Sony, TCL Android TV] brightpen:
I have 2 separate S546 TVs that were affected by the V813 update seeming to cause various CEC and ARC issues. So a big thank you to this community here for providing this software and a way to downgrade!

If it helps others affected by this, I ran into various issues trying that downgrade process, so I'll try to provide a few notes on what I figured out and the exact downgrade process that worked for me. Sorry if I got any of these details wrong, but being a novice to this scene and just trying to fix my TVs, I hope these details might help other novices understand other references in this thread:

My TVs (in the US, so North American) were I believe previously on V306 (maybe 1 was on V399, but unsure).

The North American software for the S546 only seems to be available via the "OTA" distributions, and not the "IMG" distributions. The "IMG" distributions seem to maybe only be the international software versions?

It seems like maybe the international versions maybe follow a different versioning scheme? I'm not entirely sure about this, but it seems like the international V418 (build date 2022-03-26) is actually older than V306 (build date 2022-05-18). This initially caused me a fair bit of confusion, since I figured these versions incremented (eg, I expected V418 to be newer than V306), but this doesn't seem to be the case.

I could never get a downgrade to occur directly with the "OTA" distributions. I thought I saw references to this being possible, but I never had any luck despite various attempts. Instead, what I had to do was downgrade first via IMG, and then I was able to do OTA upgrades on top of that (and for some reason, I wasn't able to go directly to V399, so I had to do V306 first).

I saw various references to Telegram and that's where I ended up downloading the files I used. In my step-by-step guide below I'm linking to posts in this thread that I think provide the same versions, but not 100% sure the files match.

So here's what I did to get two S546 TVs that were upgraded to V813 back to V399:

Download V8-R51MT05-LF1V418.zip IMG: https://xdaforums.com/t/tcl-2021-android-google-tvs-p725-c725-c728-c825-x925.4255729/post-86962461. Unzip. Place the resulting Update.img onto USB stick formatted in FAT32 (I tried both USB2 and USB3 sticks, and was able to use the USB3 stick, so that at least made transfers faster, but I know I've seen various references to USB2 only, so your mileage may vary).

Power the TV completely off (press and hold the power button the remote and then when prompted, confirm shutdown). Plug the USB stick with the Update.img file into the USB port on the back of the TV. Press and hold the physical power button on the back of the TV (not the remote) until the lights on the front start flashing. It should then replace all content and software on your TV with the V418 international version.

Next, since I wanted to get back on the North American software versions (maybe doesn't make a difference, but they had previously worked for me), download V8-R51MT05-LF1V306.zip OTA: https://xdaforums.com/t/tcl-2021-android-google-tvs-p725-c725-c728-c825-x925.4255729/post-87016381. Rename this file to V8-R51MT05-LF1R306.zip (basically replacing V306 with R306). On your same FAT32 USB stick, delete the previous Update.img file and copy the V8-R51MT05-LF1R306.zip file onto it (so you're placing the zipped file onto the USB, and not unzipping anything). Renaming the file seemed to be necessary so that the TV would pick this up as an update, rather than seeing it as older and refusing to upgrade to it.

Repeat the upgrade process above by powering off the TV, plugging the USB stick in, and then pressing and holding on the power button until the front light starts flashing. It should then upgrade to the V306 North American version.

Finally, download V8-R51MT05-LF1V399.zip OTA: https://xdaforums.com/t/tcl-2021-android-google-tvs-p725-c725-c728-c825-x925.4255729/post-88256155. This one did not require a file rename, so on the USB drive delete the previous V8-R51MT05-LF1R306.zip and place the new V8-R51MT05-LF1V399.zip on it.

This update can be done through the more normal local update interface, so while the TV is on plug in the USB stick, go to Settings > About > System Update > Local Update. It should then find the V399 update and step through doing its thing.

At this point, your TV should be on V399. But for good measure I ran the "Reset all" and "Reset shop" through the hidden service menu. Not sure if this is much different than the factory reset via the normal interface, but I saw various references to this. So pull up your Picture settings > Advanced > Brightness > Contrast. While "Contrast" appears selected, press the virtual keyboard icon on your remote to pull up the virtual keyboard and press in "6425" (if the virtual keyboard doesn't show up, I found I needed to be switched to a real input, like my Apple TV first). That should then pull up a special service menu where I then selected Reset all (pressing the right arrow to select it), and then Reset shop.

The end result should hopefully be a setup running V399. I use AppleTV's, so when going through the new setup I set them up as basic TVs and this time didn't let them connect to WiFi or enable any type of software updates. So hopefully that will prevent any frustrating updates like this in the future.

Hope this maybe helps if you're new to this scene and didn't know how to interpret some of this. But thanks again to this community for providing all of this!