Somewhat standard behavior for me. Diesel soot comes out of the tiniest space and looks like a lot when its really next to nothing. How long has it been installed? If it really bothers you, pull the center, really clean all the sealing surfaces and apply a very thin film of high temp sealant.
I just put all my stuff back together and tried to pay attention to the details to keep this from happening and making a mess. We'll see if extra attention makes a difference.
BTW I think this scenario is part of why s300's are more lamented than s400's.