Okay, so the mutagen in IDW alters DNA under certain exposures, right?
With its whole purpose initially being to mutate soldiers with stoney skin?
But we know the Utroms were using it with their last personal use being use for stasis?
Well, what if, at least to some extent, there was still a little bit of DNA altering in the Ooze used to keep the Utroms in stasis, but not enough to fully mutate them?
In a way, there is something very similar to the Utroms going into stasis in nature. It's something that
West African Lungfish are the prime examples for.
During droughts, they can survive years without food or water by burying themselves in mud and making a kind of mucus cocoon while they wait it out.
I speculate that Utroms used to be able to do it, but that behavior died out the more they expanded and got hardier against the elements of their planet. That is, until it got to the point where they really, really needed it. So the stasis that gets developed first "switched back" the gene that made them sleep for a long period of time, and the ooze itself replaced the normal cocoon (because they just don't generate enough mucus as their ancestors did to generate one one their own...probably because as they "urbanized", they first no longer needed so much, and maybe they started to find extra sticky bodies unattractive???) and kept them under longer, with just a tiny hint of any mutagenic effect that would have shut off when they were removed.
But because that engineering was only meant to be temporary, there really wasn't an antimutagen developed for the more "extreme" mutations.