ISR set up using FALLING but I still seem to see a few interrupts caused by what must surely be RISING edges

Many others have had issues with interrupts and these low cost sensors based on lm393 comparators. Performance is often improved with a cap between D0 and ground.

Take a look at this thread
https://forum.arduino.cc/t/solved-falling-and-rising-interrupt-seem-to-behave-the-same-as-change/666236/3

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