In tvOS 27, enabling AutoMix/Song Transitions causes stereo Apple Music tracks to be output over HDMI as 6-channel LPCM rather than 2-channel PCM, even with Continuous Audio Connection turned off.
The audio itself remains stereo, but because it is presented as a multichannel signal, AV receivers such as Denon no longer recognize it as stereo and therefore cannot apply Dolby Surround or other stereo upmixing. This may also affect soundbars that use the incoming channel configuration to determine when to apply their own spatialization.
The problem is directly reproducible: with AutoMix/Song Transitions ON, the receiver reports 6-channel PCM; with all Song Transitions OFF, the same track immediately returns to 2-channel PCM and normal AVR upmixing works.
Disabling AutoMix is therefore a workaround, but not a satisfactory solution. AutoMix and correct stereo output should be able to coexist. Otherwise users may simply perceive that Apple Music sounds less spacious or that Apple TV sounds worse after the tvOS 27 update.
[Re-Titled by Moderator]
Original Title: tvOS 27 AutoMix causes stereo Apple Music to output as 6-channel LPCM over HDMI, breaking AVR upmixing