I think it was by design, they just weren't thinking about how the Atmos mix would translate to different-sized speaker setups.
If you listen on a 7.1.4 (or 7.1.2, 7.1, etc) array, what basically amounts to the original stereo mix (nearly all vocals and instrumentation) is mixed to the 'side surround' speakers that sit directly to either side of the sweet spot. This gives a somewhat 'immersive' feel like you're inside the music, listening on a huge pair of headphones. Unfortunately if your system doesn't have those side speakers, all the content meant for them gets re-directed to the rears rather than a phantom center between FL & RL and FR & RR.
View attachment 80530
A similar problem occurred with The Pineapple Thief's
Nothing But The Truth Atmos mix - Bruce Soord's vocals were mixed almost entirely to the side surrounds (yielding an interesting 'inside your head' effect), but they get sent mostly to the rears in the fold-down to 5.1.4, 5.1.2, 5.1, etc.