They could be trying to take a dust bath in the pellets. This is something they do when they have lice or mites. Perhaps get them checked out by a vet. In the past I have had birds that kept trying to take baths in their food to try smother parasites. Once treated, they stopped. It's also something birds do in the wild.
Luna's been to the vet, she got a all clear and she is the only one that does it.
It's not just rolling, she purposely tosses the pellets out, does it more when its the kind she doesn't like.
Once she gets the pellets out, she sits in the bowl like a chicken in a nest.
She only does it to pellets, never seed and they are in the same kind of bowls.
There are some covered dishes that are too small for her to fit in, though they tend to be plastic.
What would happen if you left that dish there empty and used another dish nearby for pellets? (or is this sitting, a nesting behaviour)
Instead of thinking of covering the dish, her flinging out the pellets she doesn’t like could perhaps be helped by a larger dish, that could have just a single layer of pellets in it so that all of them can be seen.
I sometimes use transparent colourless bird baths on the floor of the cage, as seed holders (similar to the Coral seed tidy). They do help cut down on the husk mess - but she may also see them as nesting sites.
I tried a smaller dish before, she does headstands in it. She is a weird bird.
I ended up taking out the covered crock, she was acting too much like it was a nest.
For now I'm only putting a few pellets in the dish. I'm on the lookout for a small stainless bowl if such a thing is made.