I think I am understanding what you are saying but let's clarify it.
You have a catheter in place. If you attach a bag and have the flip flow either detached or open then there is no leakage.
If you attach a flip flow and close it then you get leakage from the penis.
If I am correct in my assumptions then, yes I am not too surprised. I also had a catheter and a flipflow valve and I got leakage if the tap was closed for a long time. Quite simply it is normal pressure in the bladder. The bladder sphincter would normally stop this, but now that muscle is having to close around a plastic pipe and there will be a little leakage, indeed a lot of leakage if your bladder is full and you squeeze hard.
I still think flipflow is a good idea, but just go to the toilet as frequently as you normally would.