When mitering large wide borders corners for queen size quilt, I always had one corner that was off needed to be fixed.
I would remeasure to see why the one border was off. I would measure the length of each size because one side could be just a 1/4" longer or shorter than the other one. I would pin base the border to see if my measurement were accurate and then sew the border on. I always use pins so that the fabrics won't shift but I am careful to pull the pin out before getting to close to the needle. Hope this helps.