The only difference between Circuit 1 and Circuit 2 is`that in Circuit 2 a parallel 2.2k resistor R8 is added as a load next to R6.
Why adding this R8 is halving the output voltage here?
I tested this in real with a scope, and the voltage remains at 12V pulse when I add R8. So unlike in simulation R8 didn't change the output voltage. In real implementation the grounds of the input side and output side are isolated.