Need help for a furnace procedure

Hello comrades,

I am making a custom furnace. I managed to make everything work but there's a weird bug. When I first open the GUI, the item smelt but doesn't appear in the output slot. I need to close and reopen the interface in order to get my melted item. DOes anyone could possibly help me with that ? If you need, here is my procedure (I know it's a sloopy, but it's my first one)

Thanks for the reading