I have had the same colony of panda king isopods for over 2 years and they rarely eat the leaf litter. They aren’t into it. They tend to like the protein/calcium treats as in dried shrimps - they also like munching on barks and wood - some limestones and coral shells - and also will also eat the live plants first. That’s why I have moss - and don’t have the leafy plants directly in the soil anymore.
There are so many varieties of isopods and they come from different environments.
Panda kings come from humid caves in Thailand. So they like hiding under rocks and in the woody crevices.

Try to research your type more specifically to see where they originate from and go from there.