I think the you may be right that the movement of the nostril is not 100% with the movement of the rest of the head, but it does not stay in place entirely. It generally moves with the head. It's hard for me to tell, especially in the two frames between people walking by.

My point about it being due to the beast leveling its head is not to say that you're wrong. Just that I believe Harmy hand animated this movement, and it's not a case of each individual frame being from the 35mm (IIRC). So in order to change the perspective of the head, he may have had to separate it into segments.