Mmmmmmmmmmmmmkay........I seem to be unable to do an extrude cut up to a cylindrical surface. I seem to remember this working in the past?

It's the hexagon pictured below: it needs to cut through the boss and not through the cylinder. If I choose up to next, it goes through the wall of the cylinder. If I choose up to surface and select the cylindrical surface, it goes through the cylinder to the far wall. I've attached an image for clarity.

Anyone have a clue they could share with me? Greatly appreciate it!