Sounds like you have sold some Black Walnuts. Black walnuts are the best growing trees on our place in Northern Missouri. I'd say we have over 1000 that are at least 18" in diameter of varying feet height.Yes it is a fast growing tree under optimum conditions veneer having tight growth rings not so much. To make veneer grade they simply need to be of sufficient size without defects in the log faces such as limbs, knots, kitty faces and no rot or blue stain indicating steel in the log. A wide white sapwood ring will be considered a defect. Some buyers may pay a bit more for prime color or dock for poor color. I don’t recall being docked for wide growth rings perhaps you have a wide sapwood ring confused with wide growth rings?