I am not sure how to apply the video here but I talked with the player who reported this to me. He had 1 stack of 2000 metal ore in his inventory. I confirmed this. Then he hovers loots it into the small furnace and split it into 3 slots causing double the amount of metal ore to appear from nowhere.

I had a similar issue, but it was a compatibility problem between FurnaceSplitter and ChestStack.

uh? I do not have chest stack, I have ABsolut Srter only I guess...