Given that you are consistently seeing issues, even after swapping cables and ATV hardware, it might be worth suspecting the sink, in this case, the AVR.
If the receiving HDMI chips/firmware are hyper-sensitive to jitter and are forcing re-clocking with the sender, it can result in lag in the data stream.
Remember that HDMI is first and foremost a format engineered for video transmission, and the 2ch LPCM support is kind just there 'because' some audio support was needed. But the LPCM is highly prone to jitter, as the rest of the HDMI D/A A/D pipeline is focused on the much higher-bandwidth video stream. This is why Denon came up with DenonLink 4 to provide a separate clock signal over the CAT5 DenonLink between player and processor to eliminate jitter on the HDMI 2ch LPCM signal.
So if the problem is jitter stability, one might try to address it with something that re-clocks the signal, such as a
splitter box or an active HDMI cable (such a fiber-based
RUIPRO). But it could well be that a different/newer processor that is less susceptible to jitter is needed.
I recently made physical changes to the media room, and re-did the Audyssey DRC and have been testing a ton of Atmos content from all sources, and no audio glitches beyond one very short timing slip on an Apple Music Atmos track. Chain: ATV4K-2nd Gen launch day order (current firmware) --> 3' Amazon Basics HDMI cable --> Denon X3600H
