It's over-exposure compensation. I've seen several cameras do that where a really bright area actually gets displayed as black pixels.
Do they do that on purpose? To make it more contrasting maybe?
It seems that the cameras have HLC (High Light Compensation) on in the camera. That is why you see the really bright areas , especially the whites get blackend out.
Is that a generic term, HLC, or specific to a manufacturer? Is it designed really just as a 'clip indicator' (like the red of a audio VU meter) and so you should never see more than just a small amount of blackening, or is the way the video clip looks typical?
No that is pretty generic among camera features. You would see it more pronounced when looking at something like car headlights at night.
Here's a marketing demo video that shows highlight compensation in action: