As the fans and electrical current grows the load on the alternator increases which in turn increases the load on the engine,
This causes the Ecu to reference a different load cell in the fuel table. Once it’s emulated you can watch the trace move when the fans cut in.
It’s more than likely because the load cell it references under load is lean,half an hour on an emulator would allow you to iron out the load cells around idle speed to ensure the afr remains stable and this will usually cure it.
The areas in the fuel table just above idle can be tricky to reference but I find it’s Vital to reach them while live mapping to catch issues like this.
voltage at the Ecu is important, as is voltage to the battery.so worth checking what you have at the battery and at pin 20 with/without the fans