Interesting, I had exactly the same issue. I think this is the first disc I've bought that only has an Atmos mix and no separate dedicated 5.1 mix.
I have a Cambridge Azur 551R amp and a Cambridge Azur BD650 Blu-ray and also a Panasonic Blu-ray. Both are connected to the 551R via HDMI but the 650BD is also connected via it's separate analogue outs. Both the Cambridge amp and Blu-ray were released at around the same time and before Atmos was a thing, but looking at the front panels and the specs both should decode the same things including Dolby True-HD and DTS-HD Master Audio. When using the analogue outs of the BD650 I would have assumed the player would decode it the same as the amp would, which it normally does on other discs, but it seems not!
I happened to put the disc in the BD650 and was using the analogue direct in and like you it seemed to be in stereo, much to my disappointment! I put my ear next to the surrounds to check and there was some audio present, albeit at a very low level. Anyway, after popping on to QQ to see what's occurring I switched over to HDMI and all is well with the world. A bit odd as both units claim to decode the same things so I would have expected it to work, but something to consider for the future. I've never owned the album before, but very familiar with all of the singles, sounds great when it works!