Hoppers pulling out of minecart chests they shouldn't be able to

I found this bug while using the automatic smelter design that Mumbo Jumbo showcases in one of his youtube videos (https://www.youtube.com/watch?v=7xzbx7vFqqE).

In the screenshot provided you can see the two hopper lines, one provides the items to be smelted and the one at the side porvides the fuel, for some reason the materials from the minecart that runs over the hoppers on top of the furnaces are picked up meaning that the fuel line hoppers get obstructed because the smeltable materials can't go into the fuel slot wich makes them stay and fill the hoppers.

The issue can be reproduced by building and operating the design shown in the youtube video.