Haven't ever heard of it being a problem. Is this the first time the engine was apart? Could it be that someone had it apart before and painted it? Looks like you have an interesting job cleaning the paint swarf inside that block. Elbow grease or light sandblasting?
It appears to be overspray around the edges. And only on the upper case. Like the paint outside it bubbled up and started flaking off inside. It clogged up oil punp screen, the bypass valve and the non return valve for the tank. Has been quite the job to clean up.