Recent update to Apple TVOS 14 appears to have disabled 5.1 audio from Home Sharing movies

I'm using tvOS 14.3 on an Apple TV 4. I have a group of media files with AC-3 5.1 audio stored as Movies in iTunes and accessed by the Apple TV through Home Sharing on a 2012 Intel iMac running the latest Mojave. I recently noticed that the audio of those titles is now being bounced down to stereo on my 5.1 digital audio receiver. The display from the receiver shows LPCM rather than indicating a digital source. I'm confident that the change is software related, because I haven't made any settings changes on any devices, and movies streamed through the Apple TV's iTunes or HBO Max apps, for example, are still delivered with 5.1 audio. The Apple TV HDMI output goes to the TV, and the audio is delivered from the TV to the digital receiver through S/PDIF. Audio Change is enabled and set to Dolby Digital 5.1 on the Apple TV.

It would require pretty extensive research for me to determine precisely whether that status might result from one or more tvOS updates or iTunes updates for Mojave. I suspect it's the tvOS because there are changes in the interface related to audio on the Apple TV that I can observe now that I've started looking into it.

Is there a way to correct this issue, or has Apple finally decided not to recognize multichannel audio input from my version of Home Sharing? Thanks.