While diffusa 'usually' don't eat plants, in my (albeit limited) experience, they can eat plants under some circumstances. It's just unusual.
Eg. for 3 months or so my snail tank was absolutely overwhelmed with the babies that resulted from foolishly letting three clutches hatch. I have two live plants in the tank, and one of them (hygrophila polysperma) was picked on relentlessly by the snails. They couldn't eat the older leaves, but the new growth was picked off as soon as it sprouted. Every time the plant tried to grow a bit, they'd nom it down to nothing.
In this case, obviously the new growth was very soft and the little diffusas were able to attack it with their rasps, while the older, established leaves were too tough. I don't doubt that the little snails gave it their all, however. In this case, the hoards were undoubtedly motivated by hunger. (I was feeding them a dozen algae wafers a day and they still weren't satisfied.)
Another experience I had was with my very first snail. I had set up a larger tank for him that included some red bacopa, which was doing pretty good. Then I moved my snail into that tank, and in one afternoon he went up and down every bacopa stem and thoroughly stripped them all of leaves. It was remarkable to see him going munch-munch-munch and the leaf disappearing into his mouth... After that, I got more bacopas but he never touched them again. I don't know what motivated him to that one afternoon of herbicide, as he was always well-fed...
Anyhow, I guess my point is that while they aren't 'supposed' to eat plants, it can happen on occasion.
If it happens to you all the time, and there's no obvious reason (eg. hunger) or they're eating hearty plants / stems, then maybe they aren't diffusa after all? Where did you get them from?