When two hopper Minecarts are placed diagonally of each other on rails their hitboxes overlap.  The game thinks the lower hopper cart is underneath the top hopper cart and the lower one will pull items from the top one.  Java doesn't do this.

This is especially annoying when needing to use hopper minecarts in super smelter systems where they will be running in this configuration. Items in the "smeltable cart" end up in the "fuel cart." Note the image included below. I just tested this in 1.16 myself and it is still a problem and has been since 1.13.