Auto smelter gives tooo much xp

I created a sort of auto furnace system back in 1.14.60, with two input chests for the fuel and input and one output chest for the output. When I updated to 1.16.20 and tried to collect the xp by locking the bottom hopper and taking an item out from the furnace. I gained the correct amount of xp the first time because of the stored xp, but after that, I still gained massive amounts of xp from just one item. I have attatched a video below of the process.