MCU External Interrupt False Triggering
Your input setup has a time constant of 10 microseconds, a huge amount of time when looking at MCU ports.

When the input transitions, it will spend significant time in the input indeterminate region - the range of voltages between Vil(max) and Vih(min). This will result in the input pin sensing something and quite possibly changing states multiple times for a single input transition on the output of the input buffer, thereby resulting in multiple 'phantom' interrupts.

In addition, this appears to be a CMOS input, and slow rise / fall times risk damaging the input buffer structure.

This application note from TI explains the issue.

Removing the capacitor allows the signal to move through the indeterminate region cleanly, minimising the possibility of metastability as these pins are asynchronous inputs.

I see the port pins have a Schmitt trigger available, but the 'peripheral input' buffer does not. If you can select the Schmitt trigger input, then the capacitor should have no effect (other than reducing the number of interrupts you may have per unit time).

The datasheet I have shows a mixture of trigger and 'ordinary' input pins, but the list will vary by device, so check your specific pin tables.