That's something not unique to the app (since it's just a web view) and it also affects the site in web browsers as well. I don't know if there's an easy way to add parent-forum navigation options to the top of those pages but I'll inquire.
The forums app was a native experience. The main app only ever used a web view for the forums. Given the complexity of the forums app, and the forums app only applying to the forums, it didn't make sense to only keep that app. There were also a lot more active users for our main app versus the forums app.