As the title says, I’ve come to believe that have a good one is a semi polite way of saying “I don’t have an interest in interacting with you further.” The reason I came to this conclusion is because when there was a teacher who was very interested in me, she’d always say things like “Have a good day!” “Enjoy your evening”, etc when she was happy to see me and would always come to chat with me. After I ghosted her for a while and stopped talking to her, when she saw me next and she was leaving she just said “Bye have a good one” with no smile. So I’ve basically concluded that whenever anyone says that to me, they don’t have an interest in my friendship or conversation and I feel a bit offended but I move on. People say it’s just a “chill way to say bye” but I disagree.