If were being entirely honest almost anything is possible in a science fiction environment, they created a cross gene human being, it literally is a spider-man, so it's not unbelievable for there to be specimens that can survive, temporarily, with their heads cut off.
You could have specimens to survive and thrash for a bit longer it's not that extreme.
However - this also begs the question - where the **** are the other biological freaks? We have the crawler and stalker - how about a regeneador/iron maiden type dude from Re4. That was probably the scariest dude throughout the entire series