steps to reproduce :

1) Build the minecart unloader like in the image (the bottom 2 are droppers, the top one is a dispenser, and the top rails are powered rails), put lava in the cauldron, put one minecart hopper in the middle dropper, then one in the bottom dropper.

2) At this point, it loops perfectly, the minecart hoppers are picked back up by the hopper, no problem.

3) Put items in the barrel

4) The system breaks almost immediately : one of the item picked up by the minecart gets picked up by the hopper dropper dispenser loop, the minecart hopper drops as an item on the rail.

I assume it is a bug in the sense that one of the minecart hopper content items is dropped in the position the minecart hopper item should appear, they basically swapped place