This is strange
The igniter and your led bulb are on the same 12v circuit! Actually, everything operating on 12volts is on the same circuit! (edited) But, The igniter will momentarily "suck" amps out of the circuit to cause a spark (or activate a glow plug , depending on furnace model), for purposes of igniting the gas mixture at the venturi assembly. IT IS NORMAL! Hope this helps. In my younger years, my business was supplying propane and repairing furnaces, etc....