Xbox One X vs PCM audio vs Soundbar

[GravityGunner]

I have just bought a Sony HT-ZF9. It supports every format under the sun, including PCM 2.0, 5.1 and 7.1.

My Xbox One X is connected to the soundbar which is then connected to the TV via ARC (not eARC, in case that matters). When playing Blu-Rays the Xbox is set to pass the audio through directly to the soundbar.

All is good with one exception, Blu-Rays with PCM audio. I have various movies, usually from Arrow, that have lossless PCM audio tracks ranging from mono up to 5.1 and also my venerable old copy of 2001: A Space Odyssey which has a PCM 5.1 track.

Whenever I play such a track, the display on the soundbar is telling me it's receiving whatever output setting I choose for gaming audio. I assume the Xbox is decoding the Blu-Ray audio instead of passing it along like it's supposed to. The only exception to this is if I set the audio to Dolby Atmos, in which case the soundbar reports it is receiving a LPCM 7.1 track.

If I set the Xbox One to output PCM 7.1, what effect on the sound would this have? For example, if I watched a movie with a 1.0 track, would it just mix that in with six other silent channels or would it try to create some sort of artificial upmix that won't sound right?

I should note that the Xbox outputs PCM 2.0, 5.1 and 7.1 to the soundbar just fine from other sources - games etc. If the track is a Dolby or DTS one there's no issue.

Thank you in advance for any advice/help etc.