Hopper moving items that don't match its contents

I have a simple furnace setup with hoppers pushing kelp from a chest through. I needed to cook some beef so I replaced all the slots of the hopper with the new material. I began pulling some of the kelp from the furnace to hasten the process. Kelp was still flowing from the chest into the furnace even though all of the hopper slots were full of beef and shouldn't have been allowing kelp to pass.