Droppers can not put items in to furnaces like hoppers can.
To recreate:
1)Place a dropper so it faces in to a furnace
2)Power the dropper and you can see no items will be moved.
Linked issues
Comments 4
I have experienced this issue the other way around, where hoppers dont put anything into furnaces when pointing into it sideways, on top of an item elevator.
Duplicate of MC-6972.
This is a problem with torch updating, not with droppers