I've noticed but not tested in detail yet.
The bikes are quite random with what they'll knock the player uncon for.

I was watching Freshspawns earlier cruising around 35kmph in the forest, clipped a stump and went from full HP to almost dead..

Then later going slower hit random collisions ( I forget what specifically ) and was fine but was knocked uncon.

It's a fun laugh a few times... but a major annoyance for wanting to survive long..

I agree. I made a gigantic jump in debug that left me red flashing. After recovering to full health and hit a concrete road edge, and died. Its one of the reasons I made the issue.