As the driver's hair is a little wispy at the top, it's actually quite a challenge for a good key, so I experimented with the Color Difference Key and Matte Cleaner and also added some Blur and Sharpen and found this bug which causes speckly halos around the edge of the mask.
I've reduced the problem to the fewest number of steps to duplicate it in the video below. It only needs Blur and Sharpen to appear.
It's present in Hitfilm 3 Express, (awkward for the course) and also Hitfilms 3 and 4 Pro - worse in 16bit float - although I didn't bother to put HF3P in the video
It's quite similar to a couple of related previous issues with (apparently) GoPro footage, since fixed in a later update (I think, I never checked.)
(MOD edit: Have edited the title of this post to make it more specific)