Digital input is getting pulled to 3.3V

I’m trying to use the tutorial here, the section on enhanced trigger logging, to log the output of a camera shutter signal. I have chosen aux 6 as the input port, and I’ve confirmed that BRD_PWM_COUNT is 4 and the CAM_FEEDBACK_PIN is 55. CAM_FEEDBACK_POL is 1. To test the setup, I have a function generator making a 5V square wave at 5 Hz, and I expect a gcs message at at rate. However, the interrupt does not see the signal.

When I attach a scope to the output, I see that the signal never falls to 0V during the low cycle, but drops to around 3.3V instead. Without the signal generator, the pin stays around 3.3V. Is there some setting that is causing the input pin to be pulled to this value? Why cant I drive the pin low?

P.S. This is my board