well, Ella said that on her 5th evo, she could regen from any tiny part of her body. Maybe thats how body evolution works. at first u cant just regen from anything and that fast, but as power grows, so does ur regen abilities grows and changes.
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