Andy W. Desplenter

If that explanation were valid then the bug would be that you do not take damage falling into webs while riding a cart.
Either way, the behavior is not self consistent and suggests that there is a bug.

Further to my previous comment, falling damage is not reset by water or vines when in a minecart, but downward motion does get reduced to cobweb speeds when falling into a cobweb while riding in a minecart.

Is this expected behaviour or a bug?

I just ran a test for something that may be related. I arranged a minecart track to have the cart fall into a one block wide channel that had vines in it.
I can jump into the channel and take no damage no matter what height I jump from, but if I ride a cart into the channel I take damage as though there were no vines.

It seems that falling carts are not affected by things that reset your fall damage.

Is this expected behaviour or a bug?