Figured out my pour over tasted like wet cardboard because of one dumb habit
I was rinsing my paper filter with cold tap water for 6 months until my buddy watched me do it and asked why I wasn't using hot water from the kettle, and yeah the cold water was leaving a papery taste in every single cup.
Oh man, six months of cardboard coffee, that hurts to hear. At least you know now, right? I did something kinda close where I never washed out my carafe and just kept pouring fresh stuff on top of old oils. Took me way too long to figure out why every cup had this gross stale note in the back. The hot rinse thing makes so much sense too, heat pulls the paper taste out instead of pushing it into the coffee. Glad your buddy caught it before you hit a full year.