I looked at your pics right after you posted. It's hard to tell from the pic but I hear you saying the problem is isolated to a smallish area and not spread over the whole upper area of the cap. You also say "painted". If it's been painted already I'd think it's a problem with the paint.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks