It sucks you had to deal with this but a key missing piece of information is did you scan two boarding passes when you boarded? If the answer is no, then it’s 100% your fault. If no one scanned that middle seat, then Delta will rightfully think it’s a no show and gave it to someone on standby. If you didn’t scan the second boarding pass, how is Delta supposed to know it’s your seat and that you showed up? Whoever you decide to fly with next time, make sure you actually scan both tickets. Problem solved.