Because it's triggered by the amp test I suggest you have a short in the wiring somewhere?

Check your flipper buttons, sometimes they get gummed ups and even though you release it, it can stay engaged a bit

Are you sure it’s not a mechanical problem? Are you able to replicate the hanging button in switch edge test? Dredd uses optos so it’s not able to be from sticky leaf switches. When I got my Dredd, it had an issue with the lower left flipper where it would hang, sometimes briefly, sometimes longer. The plunger was so worn it actuated to the point of the EOS getting hung up on the nub that engages the EOS. Any type of vibration was enough to release it. Yes, even turning off the power button.

This may or may not help. But this was my somewhat similar issue yesterday.
https://pinside.com/pinball/forum/topic/shadow-flipper-sticking-on#post-6888876

It sounds like your flipper circuit ground is shorted to the lamp matrix. Try running the single lamp tests and see if one or more of them activate the flipper. That may tell you which circuit to check for a short.

Could you have some kind of issue where the fliptronic board is reading data meant for the driver board? Ribbon cables?

The WPC system uses D0 for low right high power and D1 for low right hold. See if single lamp test on D0 and D1 lamps make it flip.

I would look towards the fliptronic. CA0, CA1, CA2, CA3, CA4, WDEN and R/W go the decoder chip on the fliptronic board that decodes out when it the fliptronic board should read the data bus. Gently flex on the ribbon cable during lamp test. If one of the address pins to the decoder floats (no pullups) you might get a situation where other noise from say lamps switching can drive the input.

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You beat me to to jump on this post. Failing decoder chip.

Yup, that's a weird one!

When that game came out, it was right around the time they had black plastic flipper interrupts at the flipper buttons. The black plastic was made of a plastic that was too flexible, and actually melted/bent when it got hot in the cabinet.

The fix was to make these black plastic interrupts out of a different material, so for a while they were made out of white plastic, and if you ran across a machine with the black ones, you were supposed to replace them with the white ones.

BUT...

The same thing you found! Reflected light would cause the flipper to activate without the flipper button pressed. So they immediately started replacing them with black... but a different kind of non-melting black plastic.

Makes me wonder if your machine has white plastic interrupts. If so the part to replace the white ones with is this:

https://www.marcospecialties.com/pinball-parts/03-9001

Good catch on the troubleshooting here!

My lower one is the same way. I believe because the bottom tab does not swing out as far as the top due to the arc of travel.

also if you replace them stay away from the white plastic ones , the light passes thru them and does not interupt the light as good as the black ones

Looks like an aftermarket PCB?

I think that is by design so the flippers do not activate at the same time. Push the flipper button slowly and see what I mean.

I believe you are correct.

In a pinch, if the black ones aren't available, you can always paint the white ones black so light will not pass through. This is a temporary fix, but will work.