Considering the problem was apparently introduced about 1 July with the Atmos update, I think it's probably still a little early to expect a patch to fix it. If there's not a fix out by 31 October, I might get a little worried.
I'm getting the same thing on my R635 TV via all streaming apps when viewing Atmos enabled content. Seems to be a negotiation issue. I thought the issue was my Vizio soundbar, but that has been RMA'd and the issue continued.
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.
I just installed the new update (24th october). It's not perfect, but it's way better. The sound and picture freeze 1 or 2 sec every now and then and it's more frequent in some movies than others but now it's watchable. Finally, i can watch my movies in 4k hdr atmos. Thank you roku for listening. Keep the good work. Let's hope for no freezes at all with the next update.