I wonder if Rue’s hormones are starting to kick in and if they are making her feel a bit crazed? She’s too young to be spayed yet, but might not be too young for the onset of teenage rampage.
Is she getting plenty of out of pen time for play and exercise? If not, maybe she has a lot of pent up energy and some frustration at being in the pen for long periods of time.
It would be pretty difficult to cover all the sides of an x-pen with fleece blankets, and she probably wouldn’t like feeling closed in by that anyway. Fleece isn’t as bad as other fabrics if they chew on it, but you still don’t want rabbits to ingest a lot of it. There really wouldn’t be a way to affix large pieces of cardboard (from boxes) to the sides of the pen either. So, I don’t know if there is any kind of barrier you could use to keep her off the metal grids. Maybe someone else here will have a better suggestin.
If Rue is getting plenty of out of pen time, this may just be a habit that will (hopefully) be stopped by her spay surgery.