Firstly check the same pressure guage on your car while parking at night after filling and then in the morning. A 10-15 bar drop is normal.
Reason for the drop is that at cng filling station the gas when compressed, it gets too hot and the chillers are used to cool it down. You may have seen then on their roofs with water running. When the cooling is not sufficient the gas at dispenser is hot. You may have noticed that in summer the cylinders capacity is reduced than in winter. Same is the case with mileage. You fill the same pressure in both cases.
Reason for your pressure drop may be:
1. Leakage (can be a faulty main solenoid valve at cng kit. if it does not closes properly, the gas will leak out of the air filter). Try closing the valve at cylinder all night and in the morning check the pressure after opening that valve.
2. Hot CNG at station. So higher pressure drop when as gets cold. At some dispensers, the temperature of gas filled is also shown. In summer it can go upto like 30 deg. C and in winter its usually 10 to 15 deg. C. Change your CNG station to check this aspect.