I believe that interleaving is the key. I see the same symptoms on planar formats, but interleaved formats work, even for 2 channel audio. I expect this is an undocumented requirement (and an Apple bug). Tested on iOS 16.1.

Thanks for the answer. The link you've given is not publicly available. I think you meant help.apple.com rather than apdreview.apple.com. But I understand what you mean. FB11797677 I'll consider whether making a custom modeler is worth the trouble here, but I understand how that would work. Thanks again.

Good news is that it does seem fixable. We created our own (slightly feature-limited) drop-in replacement for AppStorage, using PMKVObserver to handle the KVObservations in a thread-safe way, and it works fine. Feels like something Apple should hopefully be able to fix without too much trouble, and we have a solution that we believe will work for us until then.