Connecting AC analog amps meter
The output of the SCR regulator at less than full voltage will be a short positive voltage pulse and a short voltage negative voltage pulse during each cycle of the input voltage. The meter may not respond well to that waveform. The multimeter may not be accurate either, but it might be better designed for a distorted waveform than the panel meter. Inserting the diode into the circuit will convert the current through the meter to DC. That may interfere with the SCR regulator and cause it go to full output, but the diode would only pass the positive half cycles of the AC from the regulator.

You could test the meter by connecting it to measure the current through an ordinary AC load without the regulator. If it works properly with 10 amps AC without the regulator, it may work properly at higher current from the regulator without the diode and resistor in the circuit.