Bobbies can be perma-killed quite easily. Those two bobbies are just glitched that when they die their head is removed. I won't spoil how to kill Bobbies since it's quite the experience repeatedly killing the same Bobbies before they finally wear you down and kill you, repeatedly, to figure out their every weakness. Though I will inform you about one weakness/strength.
Bobbies heads are not vulnerable in combat unless both the legs are destroyed and you've used the talk action to ask them why the law is in effect and that they serve and protect the people.
After doing that the head will be vulnerable to attacks but it won't display the vulnerable mark when it's vulnerable.