Nothing was wrong with the powder. Plated bullets need a little more powder than cast bullets, is all. Wadcutters have a huge bearing surface, magnifying this effect. Add cold weather and a 6" tube, and you can run into problems.
I don't shoot any plated bullets, anymore. But when I did, I found that they varied. Some shot more like cast, and some shot more like jacketed. Ideal charge was almost invariably somewhere between cast and jacketed loads. Usually closer to jacketed data, but not always.
