It is common for the steam generator to experience a dip in steam pressure or output when water is being pumped from the water tank into the boiler. This dip in steam pressure is only for a few seconds.
It is also common for the initial burst of steam to be stronger, then taper off to a steady level with continuous triggering of the steam button. This is because the pressure will drop to a steady state level when the steam trigger is continuously depressed.
If steam output does not recover, kindly check the steam generator panel for any notifications which may be causing the drop in pressure such as the tank being empty or the steam generator being in standby mode.