I also have the same issue with my app which is SwifUI documented based app. Since Xcode 16.2, it doesn't show content view (empty) when running on Mac (Designed for iPad).

Do we have any update on this DocumentGroupLaunchScene toolbar issue? I have a similar issue on iOS 18.4 RC and RC 2. When my SwiftUI document-based app opened the first time, it had duplicated toolbar items. The second time, it opened with another document, it had more duplicated toolbar items with the wrong document name (first document name). Toolbar items flickered from time to time and didn't respond to the tap. This issue can be re-produced in the sample code (Building a document-based app with SwiftUI). I have submitted feedback for this, FB17025216.

Indeed, on iPadOS 18.4, .onAppear is called twice (on 18.3.1 and before, it's called only once). Sometimes, the navigation bar flicks and shows the newly opened document title instead of the previously opened document title, and duplicated toolbar items disappear. Still, the duplication comes back sooner or later. This issue is also impacting document saving. When re-opened, the document modification is lost (the modification wasn't saved). There must be some changes in DocumentGroup on iOS 18.4 and iPadOS 18.4.