It seems that all of my tiles in set A think they are passable, even though they are marked X in the passability settings. This is a new development. It holds true in all my maps now, and I can't think of what I may have done to cause this. No new scripts, no new anything. Any ideas?
Never mind, I got it. The first tile on tileset B was set to "O" instead of *. I'm not sure why, but I was once told this makes all the difference.