From the schematics attached it looks like the flash is not connected to output of regulator. Please correct if my understanding is wrong.
So,I am not sure how the voltage is increasing at the o/p of regulator when the flash is connected. Can you please provide the entire schematics.
If the flash is connected to the o/p of regulator , then when the flash is powered on it might be drawing current increasing the voltage at the output of regulator.