Does your decoder callback always receive -12911 or is that an occasional error? I've had issues with occasional and unexplained errors, and the only solution was to wait for (or in my case request) a key frame to get it back on track. It seemed to relate to low bitrate AV1 streams. No idea about VP9, I don't use it.

Hello, yes I did report this via the Feedback Assistant back on Oct 1st, its FB15344866. I've not yet had a response though. PS I also discovered the issue is present on iOS 18 too. The exact same piece of code can decode a 5.1 EAC3 stream on macOS Sequoia, iOS 17 and tvOS 17, but fails on both iOS 18 and tvOS 18.

So an update to this, in case anyone is remotely interested! I added some sample code to the Feedback Assistant report which illustrates the problem. However I have since discovered that the equivalent objective C API - AVAudioConverter does not suffer the same problem. i.e. the same exact packets are decoded successfully on all platforms when using this API instead of the lower-level AudioConverterFillComplexBuffer API. If anything I would have expected the higher-level API to be using the lower-level one so this is a bit surprising, but may help someone else who has come across the same issue.

Unfortunately so far the only way I have seen to get an iOS or tvOS device to output Dolby Atmos is if you use AVPlayer. I have made a feature request for more APIs to be able to output Atmos, for example why can't AVSampleBufferAudioRenderer output Atmos when given a suitable EAC3/JOC stream? At the moment it's very restricted.

No feedback here, no feedback in the feedback assistant, I don't know why I bothered.