Were the GO and GA drop targets infact down when you got this result?
If you do a continuity test of the blue reeds on the NMP board sitting on the bench, do any show zero ohms continuity? i.e. like they're reading as a closed switch?

On the lowest pop bumper, what switch(es) are reported in switch test mode when you close the pop bumper point score switch (not the spoon switch but the switch above it that the pop bumper activates for scoring)?

Is there any space to try and put a magnetic shield (metal plate) between the two as a test?

Great to hear! I see you did some research on the material to use
At least we know the concept of disturbing the magnetic field works.