I am not a chin expert but my first chin, a rescue male, started to chew his tail at a time there was a great deal of overhead impact noise. Like you I took him to the vet, got tests done and the conclusion was it was just stress. At loss as to what do (the vet did not recommend anything) I started putting Bach rescue remedy in his water. Whether it was the remedy, an abatement in the noise levels or both, one day the chewing stopped although it took a while for the fur to grow back.
That said he had previously had a bout of fur chewing caused by a level of humidity that was a bit high (I live in rainy Scotland) for creature that evolved in a semi-desert. Once that was fixed the chewing stopped as well.
So in the absence of a pathogen the root of the problem could be no more than a small environmental change that altered the balance. Do you have any other pets that may have changed their behaviour or be sneaking close to the cage? Have you tried to cover the cage or add an extra comfortable hide or hammock just in case?
Sorry not to be more precise or better informed but an 'environmental audit' may provide a solution. Hope it works.
