Looks to be Yellow. I can still see Cyan in the print after the line transitions into that other wood grain. Magenta and black also look present. Gotta be yellow.
I have seen some developer issues like this though. Not a little clump of developer that creates a hair line, but when internal metal roller fails to magnetize on a portion of the roller. creating a large dead spot. This was on a different machine though, so take it for what it's worth.