Update on the issue I initially reported on 29 July.
It appears that the Roku OS update that was installed on my 4630X circa 26 October has resolved the serious playback issues I was having with Dolby Atmos content from the Apple TV app.
I watched 10-15 minute segments from around a dozen different movies and did not experience any significant audio stuttering/dropouts, video freezes, or buffering.
There may have been one very minor audio blip/very brief dropout, and I got the feeling once or twice that the frame rate might be either slightly fast or slightly slow (not perfectly synced with the audio); however, these were minor issues that may very well have been my imagination.
I will report back if the serious playback issues return or frame rate/sync issues recur or worsen. For now, much improved performance; the Apple TV app is definitely watchable/enjoyable with Dolby Atmos.