Dear henabi, this is how the back of my wallet looks like.
I used previous brands of wallet either had a “burnt” mark or this “pimple” problem which my current wallet has.
Happens on smooth leather or grained calfskin.
I brought back my previous wallet to the shop and the sales Assoc said that could be the heat that cause these marks.
But I never put the wallet in back pocket and sit on it so unlikely heat from my ass.
Maybe it’s heat from my phone cos I put my phone and wallet is same front pants or jeans pocket or heat from my thigh