Currently, Netflix streams on Apple TV can only be played back with stereo sound. Apparently the app has a bug that only outputs LPCM 2.0 sound, no matter if there are also better variants like Dolby Atmos.
There is nothing you can do about it and we assume that Netflix is working on fixing the problem. This hint is only to give you the certainty that the error is not caused by you or any setting that you might otherwise spend hours looking for.
So in this case you should wait until the update is available and be satisfied with stereo. There are worse things than that, as you know.