From reading about experiences from others that's supposedly because of the accentuated sub-bass. For that reason they suggested exchanging the M15 APEX module for an N0 ATOM module from FIR Audio, to create a very useful step between M15 and MX. This would lower sub-bass attenuation and make mids sound slightly fuller. I thought that might be much more to my liking, tonality-wise.
Actually, thinking back about your U12t review this slight modification may actually give it the warmth and engagement you were missing yourself...
https://www.firaudio.com/store/atom-xl-4pk
drftr
I dunno I can't comment on the experiences of others but it didn't matter which of the 3 modules I had that I was using the midrange of the Nio always came off like a 2-d image atop a 3-d mound of bass...with varying degrees of bass.
It's not warmth I was missing-- my issue with the u12t isn't that it was "cold"-- it's that I found it thin, which is different. The u12t actually has a very warm-ish sound overall...it just lacks certain dynamic weight, density & grip that I like.
As with everything, YMMV.