Well, the second evolution is basically to stablish the bases of Superhuman's powers, and balancing said power. Maybe the Mc's powers weren't fully developed and his regen was prioritizing the most important parts of his body.
So when he evolved, his regen gained a boost and now is capable of regenerating from his spine, meaning that having his head blown off wouldn't kill him anymore.
It's like Danica and Valravn, in a Dead end Danica would split him and the Mc dies instantly, but when he evolved and fought Valravn, he was cutted and split a bunch of times at the same time, worst than Danica's attack, and survived, even seemingly still conscious and ready to fight back instantly.
that's fair, however, another way to look at it, in the self decapitation scene, mc stretches neck so that can also be seen as stretching spine. there is also the fact that we learn later on, mc is weird when it comes to super humans, for the sake of not saying spoilers, all im gonna say is that that might have something to do with it