Is it possible that the switch is not working?

It should Tilt the ball off the plunge if the left gate is stuck up. No other purpose.

There are a few Topics on this. Williams mystery switch.

If you go to IPDB site. Click on Glossary, scroll through, or click on T, scroll to "Tilt Roll Over Switch". There a few paragraphs on this.
Basically, if the stuck gate is open, on the ball plunge, the ball will be Tilted. This prevents multiple shots up through the spinner, hitting the gate on the right, ball deflects back through the gate, spinner, back to the flipper.