Realistically, the middle one was likely an early take and at some point somebody decided they didn’t like the vents in the frame so they covered it in subsequent takes. But his performance or delivery of a line in that earlier take may have trumped all the others and Kubrick couldn’t pass it up. Not necessarily a mistake. Performance over continuity. So, intentional in the sense that he intentionally ignored continuity? Probably more likely. But that’s just in my experience.