Regarding 15.4.1: I disconnected my gen-1 AppleTV 4k from my receiver and reconnected my 2021 AppleTV 4k a few days ago and updated it to 15.4.1. Short answer: not fixed.
We've been watching "Severance" on AppleTV+ but have been using the AppleTV+ app on Sony's GoogleTV system on my A90J using eARCto a Denon AVR-8500HA receiver. Atmos was working well there and the sound was spectacular.
After the update, I switched back to the AppleTV for a couple of episodes. I was pleased to get through the first episode with just a couple of 1/2-second dropouts and thought great! That's tolerable and maybe not the AppleTV's fault. But when starting the next episode, the clicking and stuttering returned and I gave up.
After switching back to eARC, I also noticed the surround sound was much richer. The AppleTV sounded kind of thin by comparison, with less bass and muddier dialog from the center channel. I was actually shocked by how much better eARC sounded, with crystal-clear dialogue. I can only chalk this up to the AppleTV doing the surround sound decoding instead of the Denon receiver. I'm not sure how well the Audyssey room correction works with what's effectively a multi-channel input over HDMI from the AppleTV. I switched back to the AppleTV for a bit to confirm. In particular, the AppleTV+ show logo at the beginning of a program has some fantastic, deep bass. It sounded a bit anemic over AppleTV while it rattled the dishes in the kitchen over GoogleTV which makes me think that Audyssey room correction is maybe not as effective or the AppleTV is not decoding correctly.
I will say that I enjoyed the AppleTV interface, including the "up next" list immensely and the promos were offering some movies on HBO Max that I didn't know were already available. The GoogleTV AppleTV+ app only lists shows in up next that are available in the app -- for me that's AppleTV+, purchased movies and MoviesAnywhere, along with Paramount+/Showtime since I get that discounted through AppleTV channels. Watching other programming over the AppleTV, like Stephen Colbert was fine -- but that's not Atmos.
We desperately need a pass-throuugh mode -- at this point, I can't see returning to the AppleTV full-time without that.