ATV not passing 5.1 signal

When my ATV audio is set to Atmos, it will not play 5.1 videos. Instead it automatically sets it to stereo PCM. I have to manually change the audio setting to Dolby 5.1 for the movie to play correctly. Atmos movies play fine (if Atmos audio is selected). Older models didn't do this, they played 5.1 movies correctly even with the audio is set to Atmos. I know this is a problem with the ATV because the Apple TV app that is included in my Samsung Smart TV plays both Atmos and 5.1 movies fine without having to change settings. I've tried various settings on my TV and the ATV but can't get it to play 5.1 without having to change the setting each time. Any suggestions? Seems like a software glitch. I'm running the latest version, TVios17 with a Sonos arc soundbar and a Samsung smart Tv.