The iron furnace doesn't accept items from hoppers. When putting a hopper on top of the furnace, and putting a smeltable item like ore or stone into the hopper, the item doesn't pass into the furnace at all.

Using 1.3.4.11 for 1.10.2 with Forge 12.18.1.2011

still a problem here I think, a side attached hopper puts items in output slot, not in "fuel" slot with 1.11.2-2.0.9.25

or is the hopper layout changed here intentionally?
Standard furnace has top -> top, side -> "fuel", bottom -> output

This has not been fixed yet.

Fixed in dev.