Mounting a Jaguar can be fatal.

I don't think this will likely ever be fixed. It's not so much a bug as it's a side effect of your hitbox getting huge due to the giant animation associated with getting on the Jaguar. The same thing happens when Gliding on Mir as an Evan. The hitbox becomes huge because the animation is so tall and wide, and it results in things hitting you that aren't visually close to you.