Thanks to the demo I found through another comment, I was able to recover my permissions by manually resetting them. And thanks to that guide, I now understand what Not Set (No) does. Before I only used Allow and Never.
Now I'm wondering if I'm making node permissions harder than they need to be. Let's say I have a subforum in each forum that's meant for announcements, and only staff can post in them. Does this mean I need to manually set the permissions for EACH subforum? Or is there a way I can do it all in one place?