Closing boarding early and cancelling reservations seems to happen with some regularity on AA. . . .For this reason, I am especially careful with AA not to cut it as close . . . .
Quality summary to live by. We can quote boarding pass language and CofC to each other all day. But operationally boarding usually starts early, and often ends early.
If nobody is left in the line they start working standbys (some of whom will, at that point, have higher status than you). Frustrating, and I have been caught by it too. Some things just are. (But I doubt other airlines are that much different).