Can an Apple TV 4K be reverted back to tvOS 11 after tvOS 12 beta has been installed?
If yes, what is that process, I didn't see it mentioned in the tvOS 12 Beta release notes.
You might try resetting it and reinstalling from iTunes via USB.
If it’s the newest fifth generation ATV4K no. The older ones (gen 4) have a USB-C port on the back which allows connection to your computer and iTunes and from there you would restore from the downloaded tvOS image file.