I believe there are also various natural products that can cause as much damage as chemical solvents, vinegar for example is documented as a solvent for example.
2 years ago I interacted with a customer with a similar issue and we found vinegar was involved, I am not saying this is the case here, but that was the case in that instance. It was not at first obvious since the customer had been handling something stored in vinegar and had repeatedly handled the jewellery after that.
We do sell many items (100,000+) in a year and the amount of damage we have reported similar to this is in the single digits. A dip in a chlorine pool could also be problematic, but life being what it is, recalling all the situations can be a challenge.
We warn about chemicals and we understand that in real life contact can occur, unfortunately we cannot stop it happening. I am sorry.