VG10 can rust right in the kitchen, I can tell you that first hand, and that is premium priced stainless considerably above the alloy that porche uses. 440 series will also rust and pit just sitting in a leather sheaf long enough, I have a $600+ Randall did just that. I have some cheap stainless knives from decades ago that have never tarnished the slightest, but I'd trade them in an instant for ones made of s110v, or even half-decent made 440.
Leather is acidic and shouldn't be used for storage for the reason you discovered. Leather sheaths are for when you're carrying such a knife.