Fur chewing can be caused by a number of reasons, pain, stress, boredom, habit, some just enjoy grooming & take it too far. That said, it’s also an inherited/learned behaviour. Some will pick it up from a cage mate that likes to over groom, for others it starts as something caused by something like pain/stress/boredom & becomes a habit. Then there’s those that learn it as a kit if mum is a chewer. Some it will be ingrained in by mum as normal grooming behaviour & they’ll chew from being kits, others forget about it, then randomly remember it later on down the line & pick up where mum left off.
It’s not something that causes damage as such to the chin since the fur will grow back, all be it slowly, but myself & most breeders I know usually won’t breed from chins that chew. My Goldbar girl who was supposed to be for breeding learnt it from her mum, she’s chewed every female she’s been paired with since she came to me, so I have no plans to breed from her anymore.
Unless there’s an obvious problem with your girl like an injury or illness that’s a potential chewing trigger, your vet more than likely won’t have any definitive answer for it anymore than you do, so don’t let them make you feel bad, sometimes these things just appear for no apparent reason & stick with them. As long as she’s not actually damaging the skin it won’t hurt her.