Problem occurs when people let the oxidation build up on the case and it starts to attack one part of the alloy makeup.
Various types of bronze alloys, but are any resistant over time?
Maybe that's why the Panerai SLOB was PVD coated to prevent this from happening
Panerai jumped on the bronze bandwagon when others started doing it.
Some let it patina to a certain point, but have seen others who let it go a little to far and would never buy a secondhand watch after that
Main thing. OP should have returned the watch to the seller as Panerai will probably not replace the case. They never have and blamed the owners of creating the problems in the first place.