tvOS 18 doesn't provide passthrough of multichannel audio for streaming apps offering content where it it promoted as available. This is true for devices for which the functionality existed before the 18.0 tvOS update. What's more, the 18.1 Public Beta did not provide a resolution for the issue. All streaming apps appear to be affected. Notably, Home Sharing does not appear to be affected, and continues to provide multichannel audio as it did before the 18.0 update.
It has just come to my attention that in addition to Home Sharing, the YouTube streaming app has also retained the ability to stream 5.1 audio since the tvOS 18.0 update was released. Because I have also installed the tvOS Public Beta 18.1 in an attempt to find a remedy, I don't know whether the 18.1 Public Beta release is required to allow the YouTube app to stream 5.1. Here's an identifier for a YouTube video that can be used to demonstrate: v=ijSYp6N5Yrs