From:
Netflix Atmos Issues and Problems.
And then further in the thread, there is confirmation that if the ISP is throttling, or the connection is 'flakey' (which I interpret as having bufferbloat), the Atmos audio problems surface.
I know my ISP is putting some kind of cap on Netflix traffic, as my Fast.com test (which use the Netflix servers and CDN) results are below 200Mbps consistently for a couple of months. I was getting a full gigabit on that test pre-pandemic.
Tests on DSLreports.com and against my own netperf servers (one of the perks of running a networking company) show a full 800Mbps+ with A Bufferbloat grades.
Now, the reason I have good results is that my line has effective traffic management in the router, part of which is optimized upstream flow handling. Yes, streaming is primarily a downstream flow, using up to 25Mbps worth of capacity for a 4K UHD movie, but the upstream is critical to the consistency of the stream. Pretty much all flows require acknowledgment (ACK) of received data, so for every n packets down, an ACK is sent back to confirm receipt. This ACK is used by the server to determine the pace of the link. If the ACKs are erratic, the server assumes the line can not sustain the current rate and downshifts the rate.
It is during these downshifts (and upshifts) of pace that the Netflix App on the ATV4K gets confused and we get the wrong audio issue.
My traffic management does two things with ACKs:
1- They never wait, they are high priority in the queues
2- On highly asymmetric lines (gig down, 35Mbps up) it performs ACK-thining, throwing away redundant ACKs
Traffic management ensures that both directions have sub-50ms latencies (typically 24ms avg on my line), and small, frequent packets are prioritized (control packets, syncs, acks, VoIP, etc.), also respects DSCP and uses ECN (Explicit Congestion Notification).
So in my case, the Netflix server sees a very consistent rate of exchange on flows and thus does not make changes in sending rate/resolution. Thus, I do not (or rarely) experience the issue.
My bet is that if you measure your line (using a wired connection to the router) and have a Bufferbloat score of C or worse, that's part of your issue. You can check that here:
IQrouter