Sorry, it was my mistake, please close this bug.

I have a have a simple setup for smelting 27 stacks of items with 18 buckets of lava (9 furnaces, input connected with hoppers to 4 double chests and one single chest, output connected with a row of hoppers to a single double chest; after putting in 6 stacks of items in each input double chest and 3 stacks of items in the single input chest, I put 2 buckets of lava into each furnace, first bucket is emptied immediately and transported to the output chest. so I can put in the second bucket immediately; sorry for the lengthy explanation, but I hope you get the idea).

This used to work fine, but now I noticed, that this setup doesn't change what it's producing , I had smelted netherrack bricks before, and later on, I wanted to smelt sand to glass, but the setup still produced Nether bricks, even though the input chests were filled with sand. When looking into the furnaces, they were filled with netherrack, and this didn't change. Only after I manually took out the stacks of netherrack in the funaces, sand was beginning to fill in (before that, the sand was magically converted to netherrack, I lost half a chest of sand this way)