Have you ever emptied the cylinder? I mean have you ever waited till the gas ran out?
My Civic used to give me 65km per cylinder and the red light would start blinking. I tried every tunning but to no avail.
In the end, I was asked to empty my cylinder to get it cleaned as they thought it may have oil in it. And I was surprised to see that the car ran another 40km with the red light on.
So the problem can be in the CNG guage as it was in my case.