UnloadScene Working but not UnloadAsync?

I'm working on a project with additive scenes. The process involves an ever-present Persistent scene loaded with one other active scene. When switching, the Persistent scene stays, the active scene is unloaded, and a new active scene is loaded. The methods I use are SceneManager.UnloadScene and SceneManager.LoadSceneAsync. However, UnloadScene is an obsolete method, replaced due to Trigger issues and the like, I believe. But whenever I try to use its current counterpart, SceneManager.UnloadSceneAsync, it loads only my Persistent scene and drops the new active scene entirely until I try to make the switch again. UnloadScene works the first time, so that's what I'm using for now.

Does anyone have an explanation for this, or a suggestion on how I can substitute Unload with the (I assume) more stable Async version? Thanks for any help!