That looks like an acidic wheel cleaner has eaten into the lacquer/paint.
What do people use to clean them?
Brake dust eating at the lacquer, then using these dwelling acid cleaners on them (wonder wheels is a really bad one), allows the acid to get right into the holes made by the brake dust and does damage.
Maybe it's thin lacquer, but even thin lacquer doesn't just 'melt' away... that looks like chemical or abrasive attack to me.
Dave