No oil to top end

It's not all that uncommon to have a bypass stick. Usually it is caused by debris between the bypass valve and the bore in the housing as this is a close tolerance. It doesn't take much as far as particle size to make it stick. Sometimes you will nitice the bypass valve sticking before you notice that a cam lobe is going.

Pump inlet screens are not designed to filter out small particles capable of sticking a bypass. It is the job of the filter to remove these small particles from the system. Unfortunately these particles have to pass through the pump first to get to the filter. That's just how the system works. If you put a filter or screen fine enough to remove those particles on the inlet side of the pump it would be too restrictive and the pump would not pump oil. The screen on the inlet side of the pump is intended just to keep out the big chunks that could lock up the pump.