I have now fixed this problem.  You need to go to Xbox settings - devices and streaming - Blu-ray and then untick the setting called ‘Let my receiver decode audio’.  Now Sonos confirms that it is receiving Dolby Atmos despite the Blu-Ray having a DTS soundtrack.

Have just been doing some testing with my Xbox one while watching Deepwater Horizon UHD, which has a Dolby Atmos True HD soundtrack. In order to get DTS soundtracks to play using the Arc it is necessary to untick the option under the Xbox Blu ray settings called ‘let my receiver decode audio’. This will make the Xbox recode DTS to Dolby Atmos on the fly. However, when watching Deepwater Horizon I noticed that my Sonos App was stating that it was only receiving a lossy Dolby Atmos signal. In order for Sonos to receive a True HD signal it is necessary to reselect the above option under the Blu ray settings. In essence, this means that when watching Blu rays i will need to change this setting depending on whether I am watching a movie with a DTS or Dolby Atmos True HD soundtrack. No biggy, but something I thought I would share here anyway.

I’m watching Terminator 2 dtshd via my Xbox one x, why is it that on my app it showing Dolby Atmos, is it Atmos that I’m listening to?

Probably not. An Xbox One X cannot convert a DTS-HD Master Audio track to Dolby Atmos. More than likely, the Xbox is just converting the DTS track to Dolby Digital 5.1 or multichannel PCM. If I had to guess, I would say it is more likely the former. Why the Sonos app detects it as Dolby Atmos, I have no clue.