Off their website:
11) Why don’t you have an Anti Drain back Valve?
Many filters have a rubber flap over the intake holes that is an attempt to keep oil in the filter and minimize the time it takes to fill the filter when you start the engine (they call it an anti-drain back valve). A test you can perform to see how well this “valve” works is to fill a filter with oil, plug the large center spin on hole with a bolt, lay it on it’s side and then see how long it takes for the oil to drain past the rubber flap. We have found (and have read other published tests) that the filter eventually ends up with the same amount of oil in it as if it didn’t have the flap, it just takes longer to drain out. If the flap worked as planned it would be beneficial to have the extra oil in the filter at start up. On the other hand, that flap covering the intake holes is one more restriction the oil has to get by to get through the filter and back to the motor. All of our testing shows the K&P Engineering filter with it’s ultra low resistance to flow builds oil pressure in the engine components faster than a paper filter that has the anti drain back.
I've not seen any issues with mine.