Malstrain genestealers are even weirder than the official lore and I love it.
A genestealer cult got its claws into a Hive city and the rebellion was successful, but they got quarantined and there's no incoming Tyranid fleet. The cult is locked in, no one's answering the Call home, the rebellion is stuck on Necromunda. Nothing ever goes right on Necromunda.
Somehow, evolution found a way to exploit a new niche: genestealers. Malstrain genestealers are those cordyceps fungi that turn ants into automatons who infect the whole nest. Creepy as hell, I think it was The Last of Us that used the idea. The malstrain models have similar protrusions to a cordyceps infected insect. Malstrains are very explicitly NOT a regular genestealer but something else. Malstrains are a contagious disease that predates on Tyranids.
This is not supposed to happen, Tyranids are a super evolved predator that adapts to and devours everything in their path. The whole fungus/disease aspect would point to one of Nurgle's machinations, wouldn't it? But no, there's no evidence of Chaos corruption in the Malstrain and the existing Chaos cults have no genestealers. Tyranid biology blocks out the Warp, so Chaos may not even know the Malstrain exists. If they did, the Death Guard would be on their way to pick up their successful new bioweapon. There's just no overlap where Tyranid poxwalkers would slot in perfectly.
Something on Necromunda simply evolved to consume the deadliest xeno threat, but it's just chilling with the Gym Bros(who have the real deal Space Marine physique) and the Lesbian Separatists(who do quite well as feudal lords).
All the 40k assumptions go out the window the moment they hit Necromunda and I love it.