Yes, this is perfectly normal to have 1-2 cups of black tank water to drain when removing the cap. The waste valves are not perfect. To prevent this small amount from draining when you open the cap you can install a Valterra Twist On Waste Valve
However, if you find a gallon or more waiting for you when you remove the cap, then you should have your waste valve seals replaced. Click here to make an appointment.