This is my theory about surround and immersive formats. In a small room, they may help to mask reflections even if the surround/height channels are only being used for reverb. But I have no hard evidence of that, just a hunch that the direct sound from the surround speakers is likely to arrive before the reflections, and at higher energy.I would tend to agree on the hypothesis that it may be the reduction of certain beneficial reflections too much that's limiting your satisfaction. I find it even likely the case in my room where I have mainly 4" thick acoustic foam absorbers scattered around in the walls. Which is why, far often than not, I prefer to have some kind of upmixing set in place to artificially bring back some semblance of this synthetic, room-induced sense of envelopment -- but the advantage here is it is also controllable with DSP, so I can vary the strength/amount I want without having to physically move acoustic panels around.
*try adding in a pair of KH80s as optional surrounds you can experiment or A/B with.