Push the score motor by hand. Once it leaves the home position it will rotate by itself to find home again. See if it does this when you do that.

I suspect your issue is manifesting itself every time the score motor runs, and is not directly linked to the 50 points action. I've had that happen a few times with different features on different games. It needs the motor to run to manifest the problem. Rolling over the B-I-G letters does not run the motor, nor does any 10 point scoring, etc., so those don't let the problem show itself. But 50 points does run the motor!

The 50 points is likely not directly related and is just the thing that happens to run the motor the most. (Big Ben is pretty plain, 50 points may be the sole thing to run the motor DURING GAMEPLAY)

Run it by hand (push it with your finger gently - it doesn't take much to push it off home - you are not trying to rotate it all the way yourself, you are trying to get it to re-home itself with a gentle get-going push off of home) and it should do the same thing, and then you can eliminate any blame on the 50 points circuit.

In this case there is nothing wrong with the score motor switches either, rather something up or down the line - almost always a single stuck switch on a relay.

The relay to reset the B-I-G targets likely has one switch to reset the targets through the motor, and one switch to actually run the motor, so on and so forth. In normal operation, when triggered, it closes all the switches in its relay and runs the motor and completes the action. It closes the switch to actually do the thing, and also the switch to run the motor to let the action flow through.

In your case, the "do the thing" switch is stuck closed all the time on that relay, even when we don't want it, but the motor switch is NOT and the relay itself is NOT activated and is NOT to blame. Just one switch forced "on", in an otherwise "off" relay. This switch expects to be closed along with the motor switch in the same relay. It is not right now. Since it requires the motor to cycle to have any effect (not just policy - it needs something only the motor can do), it doesn't manifest until ANYTHING runs the motor. Anything that gets the motor to spin will set it off and give it what it wants finally, and it will stack itself on top of whatever real reason the motor is running that time for (aka 50 points). Like a leech. And since it is STUCK closed, it is an endless problem until corrected.

If they were both stuck or something was locking on the relay, the game would loop this action endlessly.

However, since only the one switch is stuck and the relay itself is probably not to blame, it "rides on" whatever happens to run the motor to manifest the problem.

I've had this happen several different times on different games. An interesting quirk.

Your game has a stuck switch that runs through a normally-open switch on the score motor, so it's invisible until the motor runs at any point. That's my guess... and I've seen it more than once.

If it doesn't do it when you make the motor run by hand *****WITH THE GAME RUNNING AND BALL IN PLAY*****, ignore my answer. But make sure the game is the playing state when running the motor or you will get incorrect and useless results.

Think of the relays like selector switches, and the score motor like a button to do the thing selected by the switches... a "compute" button. Pinball machines are literred with switches that run the score motor and the motor doesn't know why it is running. The relays set it up for what they want it to do. When a score motor runs with no relays on it should be "fully dead". All the switches on there are useless and dead (or possibly live and further down the chain is an open line, same thing, different layout/order) until something further up the chain sends something through them via a relay clicking on, otherwise the game would be freaking out every time the motor ran for any reason. (50 points, reels resetting, bonus drain, etc.)

The motor should only be peforming one action at a time, even with dozens of different switches on it. You have one relay properly performing an action and running the motor, and an incorrectly adjusted relay permanently performing an action, but not incorrectly enough to also have its "run the motor" switch closed, which is why it relies on any other relay running the motor to perform the error. But the relay itself is not locking on, otherwise its motor switch would be closed too and your score motor would be stuck on... I can tell from here. It's just a single switch in an otherwise-off and proper relay.

Things like 10 points or 100 points are singular and are hardwired to save time and energy... the score motor only runs when "counting"/multiples/a looping action/a string of actions is required (50, 500), which is why you're not seeing everything else running the motor aka manifesting your problem. 50 points is not the source of the problem, rather just what happens to be wanting to run the motor innocently. (And yes, they will stack! I bet you are getting those points successfully)