As a temporary solution while you figure out why she's liner diving you could put a second fleece mat on top of the first and the absorbent mat and see if she only goes under the first one.
As for stopping it entirely, it could be that she wants to burrow and doesn't have anywhere to do so. It is a natural instinct so a dig box is a good idea! Pebbles adored her dig box (it was just a cardboard box filled to the brim with cut squares of fleece). You could also use plastic bins or enclosed hideys like a chinchilla sand bath box. If you already have one you could also try a rock box to help her get that natural burrowing/digging instinct kick. Once again a container of some sort with raised edges, a layer of smooth, flat, stones big enough that she can't eat them, and bits of food or treats hidden between the rocks.
She may also be bored or not like her hideys! Trying to swap up her cage layout and switch out some hideys or toys for others might help too. I've heard good things about fleece forests!
One last thing that I doubt but that could be the problem is if she isn't warm enough or if she is too warm! It's probably more likely that she's just bored and wants to burrow though

Sorry for the ramble lol hope this helps!