Oh this happens all the time. When Apple announced they would be scanning everyone’s private iCloud data for CSAM, they had some “PSI” system which would at some point consider the content of a grayscale and reduced quality version of the image.

The problem is that security researchers for years have known about pre-processing attacks where photos which appear as one thing (a dog in a yard) appear ad something completely different (a cat on a couch) once put through machine learning pre-processing.