A possible way to interpret it since you said you both have ADHD, is that they got so invested in the argument that they literally don't know what their original stance is. Then they just accept whatever "feels" correct in the moment and assume that is what they have been saying the whole time. I used to get frustrated by someone a lot because it felt like they where always complaining. Then I found out they have a degenerative condition and they are almost always in pain. This understanding changed my view on their behavior. It's possible the person has a limitation where they can not keep track of things while be emotionally elevated. Anyway, that's just a guess with very little data.