What paper are you using? I've really never seen that before with my 11880's. I use only PK papers, mainly exhibition fiber, luster, and Ilford Galerie Gold Silk. We crank a lot of work out at the shop however with one and use a pretty wide variety of papers, including a lot of matt and again haven't really seen that.
That's one other thing worth mentioning about the 11880 to the original poster, if they like using matt and photo black inks, the 11880 is the one Epson printer that has both of those inks to the head full time ... there is no ink swapping at all on the 11880. That's one reason we chose it for our main printer at the store.