Not being able to fall through platforms is intentional design, the levels are designed around this behaviour and without it a lot of them would be very similar. I was careful to remove any places I noticed you could get stuck and not see it coming, but it's not unlikely that I missed a couple.
Finding tiles on later levels being tedious is a fair point, it would probably have made sense to include a unique visual state for 'found but empty tile' the same way there's one for found tiles that are part of the image.
Thank you for the feedback!