Plugin Imports in One Project and Not In Another

So I'm writing a native Android library. It's completely barebones. Just a public class with one static function, but the specifics don't really matter. It fails to import in one of my projects. Specifically, there are no errors, but the Project View icon is the small "page" icon that's used for an unknown file. The inspector, which normally shows a list of platforms with checkboxes when you select a plugin, shows nothing for my plugin. It's an AAR.

However, if I put the exact same plugin into a different project, it imports fine. It has the larger plugin icon in project view. It has the list of platforms with checkboxes in the inspector when it's highlighted. More importantly, I can retrieve the class and call the function from Unity. It's perfect. I can't find any difference between the projects other than the one where it fails has some other plugins. But Unity doesn't throw any errors or highlight any issues. There are no reasons that I can see why they would clash. They don't share namespaces or anything. There can't be any missing dependencies(?) because the project in which it works is almost empty. Just a barebones test project.

Both projects are set to Android, of course. Both projects are set to the default (internal) build system. Both projects are set to Mono2x backend. Device filter is set to "fat" in both. Both projects have the same minimum SDK version set (15) although I've tried changing that to match the plugin (which is 10) and that doesn't make any difference. I've tried putting the plugins in different locations but it doesn't seem to matter. Which makes sense, since they don't need to go in special folders any more.

What possible reasons are there for a perfectly valid plugin to show up as an unknown file type, no plugin properties in the inspector, and simply be ignored? And for it only to happen in one project and not in another?