Substantially different lighting depending on lighting quality?

So something really weird is happening in my scene.

If I bake at Preview level quality, the scene looks pretty much the way I want it; not too dark, no blotches or other lightmap weirdness. However, it’s of course lower quality than I want in a finished product.

If I bake at Production or High level quality, the scene suddenly gets super bright. Like someone took the exposure and dialed it most of the way up. You can still see things but it’s all really blown out. I also get these blotches all over the walls and a few other assets that basically make them look like they’re moldy.

What on earth could be causing this?