tankless water heater

I would like to have the advice. The licensed plumber installed the tankless water heater. It worked ok. Later, we noticed the city permit was required. We asked the same plumber to pull the city permit. We paid the extra $ to the plumber for pre inspection and they did. Today, the city inspection stopped by our house and saw the drawing which our plumber wrote, and pointed out our gas pipe size is not enough for the tankless heater. The plumber was onsite, but could not explain to the city. So, we could not get the approval.

Is it normal the licensed plumber does not check the pipe size before installing? The pipe is above ground, they can easily check the size.