Disassembled my pump head for the first time last night after brewing. It was mostly clean, the only issue was that on the round metal part of the impeller assembly, where I assume it is magnetic, there were evenly spaced, 3mm wide lines of black crusty gunk, in-line with the bore axis, which appeared to be dried or burned on. A non-scratch scrub pad did not getting them off with reasonable difficulty, so I set it aside for the night to look at later.
I don't find any pictures of anything similar with a couple google searches. I've always recirculated PBW followed by water, and have done about 6 or 8 batches on the system so far. I may start heating the recirculation to 180F as some of the R-eBIAB systems suggest for CIP to see if that prevents it in the future.