Is this reproducible? I have encountered this map before without any problems...and I usually just kill the rat with other weapons instead of using the HHG on it. And I don't remember any changes being done to this encounter except for changing the placement of the car if you have it.
I tried it 3 times and the result was the same but killing it with normal weapons works fine.
I don't know if this has any implications for the bugs but I'm not running the humongous version, I'm running the large version...my disc won't install the humongous one, it stops 3/4 of the way there.