welcome to ft

definitely an odd one. i suspect it wasn't cancelled before the day, but it was retimed which may have cased the issues you mention - i.e. some folks on codeshares seeing it as cancelled (post 527 onwards) FAQ : Reason for flight delay or cancellation - 2025 edition

if it had been cancelled before you checked in or went thro security, you would have been prevented from going airside to start with.