Cats losing fur due to cat litter?

My wife and I are happy parents of four cats. We’ve recently noticed our calico and our void boys have lost a bit of fur on their back legs and lower belly, and are trying to figure out what the cause may be.

They all seem healthy, regular appetite, behavior, and we’re not seeing signs of over-grooming or anything. Our current theory is it may be an ongoing mild case of contact dermatitis and the common factor between the three would be the litter box.

Has anyone else run into this issue, and if so what did you try? We usually use Arm & Hammer’s multi-cat litter (the clump and seal stuff) since it’s less dusty.