Discoloration of Gray polyester over aluminum

Q. We have been powder coating parts for a local sign company for years and they have recently returned some parts that have significant discoloration. The panels are roughly 2' x 4. There are cutouts in the parts and the discoloration did not happen around any of the edge cutouts or the edges of the parts, only the areas in between the edges that maintained their color. The only exception to this was discoloration on the opposite sides of some weld studs. The discoloration is significant, think RAL7013 applied and discolored to RAL7001. No peeling, no noticeable edge between the parts, no exposure to UV although it is a UV resistant powder.

At a loss.