Wondering if you painted over some kind of smart doorframe that changes its own mind. My old shed did something similar, but it was just bad paint and morning light playing tricks. Maybe @margaret_ward92 is onto something with the sealant idea, because mine finally stuck after I used a totally different primer. Could the sun be heating that spot up and making the old color bleed through? I felt like I was losing it too, so you're not alone in the confusion.