Yes, when the radiator fan kicks in, it draws Alot of current, which the alternator tries to compensate by increasing the actual mechanical load placed on he engine. Your alternator have a regulator which also affects the alternator 'load' on the engine. And it seems like there are 2 things that are happening now.
1. When your fan kicks in, it draws damn alot of current, creates a voltage drop which the alternator tries to compensate by increasing the mechanical loads placed on the engine
2. When the mechanical load from the alternator increase, your engine itself is not ready for it, the ecu does not expects a sudden increase in load and thus your idle drops as it cannot sustain the excess load from the alternator. The drop in rpm, causes the voltage to drop EVEN lower, below the threshold of what your meter requires to operate at probably.
When you are driving this is a non issue as the engine is already at a higher rpm and relatively high load from actually powering your car, but at idle, the power the engine is producing is only just enough to sustain the idle and any change in mechanical loads will affect the idle, unless the ecu is able to compensate for it, either by opening the TB more if it is an ETB or opening the idle valve more, but this requires an input to the ecu.
The ficd helps by allowing more air to go past the tb, it's like opening the throttle slightly in a way, to compensate for the increased mechanical loads placed by the alternator when the fan kicks in. Just like how it compensates when you turn on your aircond because the compressor places additik al mechanical loads on the engine. Same goes to the alternator when there is an increase of electrical loads
My car now with 2 SPAL fans sounds like a jet taking off whenever the aircond turns on or the fan turns on due to temp