So long as monsters and human enemies behave reasonably while alive I cannot give a flying fuck what happens to their head in the instant of their death.
It is incorrect, but sometimes a little exaggeration is fine. They are *done* at that point anyway.
In that case all bodies should just explode in a HUGE fountain of blood in the moment of their death.
Because fuck realism...
And "done" is relative: you can always change things, even post release. Especially if it's only graphical/physical stuff.