Actually this makes a lot of sense, and isn't necessarily different from your experience. I am now convinced more than ever that the InkOwl carts I had had restricted flow. If the ink was unable to partially fill the air chamber section, that means it was unable to flow through easily. That would also mean that when inserting the cart into the printer, because the ink would be unable to flow back as the manifold nipple inserts into the cart, the ink has only one other direction to flow, and that's down through the head. That would explain the dribbles you were getting.
Now for a completely hypothetical, impractical, and unadvisable idea. I'm going to assume that your R2880 worked well with the Epson carts because there were no flow issues. At the moment I cannot remember if you ordered new R2880 carts or not, and I also remember IJM doing some like testing with an old R2880, so I'm not sure if any of this applies. I wonder if filling your R2880 carts flushing with piezoflush, then leaving the head and carts filled with piezoflush for several months, doing a head clean once a week, would free up any of the flow issues.
I remember when inspecting the nipples of my R2880 manifold, I saw that only 1 out of four holes in the nipples were open. If this condition exists in the head, and also in places in the carts, it probably wouldn't take much to block the flow. Unfortunately I no longer have a working R2880 to test any of this on. Of course, it's not particularily worth doing either. But, pigment ink in a cart could create restrictions in the cart itself. My experience shows that it doesn't take much of a restriction to cause major problems.