OK - it is possible that leakage thru the isolated mos switch is responsible, you need a gate to source resistor (say 1k) and possibly a 100nF caps G-S also, to keep the gate below 2V for applied step voltages - unless you are trying to fire it - whereupon you provide a longer pulse.
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As to the SCR, when you put a new one in you are putting a step voltage on the device and it is self triggering, this is not uncommon for sensitive gate SCR's, usual cure is again a resistor, 100 ohm in this case say, and a 470nF cap from gate to cathode(k) ,assuming cathode gate SCR which almost all are these days, this will stop the SCR turning on when the battery is applied, or a new bulb, and should give you the operation you initially desired.
We use SCR's ( the TYN640 ) for firing large DC pulses for ignitor circuits for un-exploded munitions detonation, with a peak firing voltage of 420VDC and peak currents of 250A - all in a TO220 package