That tracks with what I keep running into with coffee in general, the stuff that looks or smells the strongest ends up being the thing you have to water down the most. Same deal with hot sauce, same deal with whiskey, same deal with people honestly. My guess is dark roasts get most of their flavor from the roast itself, so the longer you soak it the more of that burnt, ashy stuff you pull out, and there is nothing left to balance it. Sounds like you found the sweet spot backing off both the ratio and the time, which is two knobs most folks never touch at once. I do wonder if the grind size matters as much as the ratio here, since a coarse grind at 1:8 for 18 hours might get you somewhere even smoother.