How's today going? Well, I discovered I had some technical issues on a video I recorded yesterday so I had to redo it from scratch. And then the new video had the exact same issue, but at least with two data points I've managed to track down what the problem was.
I had OBS set to constant bitrate instead of constant quantization parameter. It's never caused me a problem before, but the snow is so variable it overwhelmed the encoder and deep fried the whole frame. Ah well. Third time's the charm.