I know a person who showed up 55 min before departure on a confirmed seat and the agent refused to check the bag at the counter. The supervisor went as far as rebooking him on the first flight the next day morning despite showing her his ID thinking he wouldn’t make it thru security. My friend said “sure, whatever,” and walked straight to KCM with his gigantic military roller duffle, walked to the gate, and listed for the jumpseat. They cleared him (probably into his confirmed seat), and he checked his bag planeside no problem.