Failed diode is allowing stray signal into one of the ICs that control the switch matrix.
Got an oscilloscope? Might come in handy to look at the signal on various legs.

Hiyas!

This is a good one!

We've got a couple of possibilities,

Likely:

The EXTREME power draw of four flipper coils at the same time is pulling other voltages low, causing the most voltage sensitive things to malfunction.

Less likely:

The holding in of the coils is producing some weird 'fifth harmonic' electrical chatter, causing weird, and difficult power drops which cause electrical circuits to malfunction. (I mention this because when slot machines first went to switching mode power supplies, the designs didn't include power factor correction (PFC), and there was a fifth harmonic that when you had a hundred slot machines all operating switching mode power supplies on the same power line to the building was causing malfunctions... basically starving the games of power for a fraction of a second...)

Maybe:

A bad diode/mosfet is letting some electrical noise when the coils engage 'spiking' the electronics.

So, a little more information, is it when all four coils pull in there is a singular event that causes the display to go weird, and a one-time pull in of some other coils? Or is it related to the hold voltages?

In either case, this is one I'd talk to Stern pinball technicians about. They might have run into it before.

If it's the first option (all 4 flipper coils are dropping the voltage for the game significantly), I would first examine the power delivery to the game! Is the power plug correct, is it plugged into a power outlet that doesn't have too many other things on the circuit, is the house wiring adequate (no aluminum wire) for the power draw?

The general usage of power for a modern pinball is 2-4 Amps, with 5 Amps peak being a good rule of thumb. That means that a 15Amp breaker house circuit can only run three pinballs reliably... not really. Six pinballs per 20Amp breaker is what arcades do.

I would REALLY examine the power delivery system. World Poker Tour pinballs are kind of getting old, like 90's model Williams-Ballys, and the general 'you gotta fix the reset issues' troubleshooting steps for WPC pinballs is going to be in my mind as I check for tarnished overheated connectors, maybe bridge rectifiers, maybe capacitors. The whole power delivery system.

Keep us in the loop, and let us know what you find!

That is an awesome catch, and one that I wouldn't have guessed!

I've commented before that I've found several problems with SAM boards running flippers... All with prior work, and using non-authorized 'compatible' Mosfet devices. Use the IRL540's for replacement!

This is the first time I've heard of the Mosfet doing something weird and causing unusual power drain. Mostly it's weak flippers or flippers that get weak.

Definitely one to remember.

Great work troubleshooting here!