I too am having issues with tvOS 18. I have several AppleTVs 4Ks in the house. Luckily only one has updated to tvOS18. I am experiencing the same issue with the video getting choppy and the audio gets out of sync. I am Ethernet connected. Tried the basics of deleting and reinstalling the app and have gone as far as resetting the device. Nothing has worked to resolve the issue. Changing the resolution does nothing. Content matching does nothing, charging the chroma does nothing, refresh rate, nothing.
In other apps, I notice some jitters and shadowing. Amazon, YouTube, Netflix all experience this. Also experience these issues in Movies in my library from time to time.
This seems like a codec issue.