Minecarts will not pickup mobs, mobs will only enter minecarts when pushed into them

When you push a mine cart into a mob the mob is supposed to ride the mine cart, but that is not happening. When you push the mine cart into a mob the mine cart just stops. To get the mob into the mine cart the player or a piston must push the mob into the mine cart. Pleas fix this it breaks a lot of stuff with mob farms. I did a search before posting this and I think that I found some else who has seen this problem but there is a thing that says the problem is solved in 16w21a but the problem is not fixed it it still there.