A barista in Portland told me to grind coarser for cold brew and it fixed my bitter batch
I thought finer grinds meant stronger coffee, but she explained that over-extraction during the 18 hour steep is what makes it harsh, and my next batch with a coarse grind tasted way smoother.
Does it really matter that much though? I've been doing the same coarse grind for years and honestly the difference seems pretty small unless your beans are super dark. Maybe your batch was just off for other reasons.