When a golden hopper inserts fuel items into the side (or front or back) of a furnace, the item disappears from the hopper's inventory but is not added to the furnace's fuel slot. The item is (apparently?) permanently lost.
Other observations:
The problem (attempting to insert fuel item and losing it) occurs even if the fuel slot of the furnace is full or contains a different item.
Normal iron hoppers behave as expected (fuel items inserted into the side of a furnace appear in its fuel slot)
Non-fuel items behave as expected (neither golden hoppers nor iron hoppers attempt to insert non-fuel items into the side of a furnace)
When a golden hopper inserts fuel items into the side (or front or back) of a furnace, the item disappears from the hopper's inventory but is not added to the furnace's fuel slot. The item is (apparently?) permanently lost.
Other observations:
Environment: