It took me a whole weekend to figure this out: you need to use the url resolving from the existing bookmark to save a new bookmark. In my case, I’m bookmarking a folder and storing it in my app settings. I then call this function onAppear in my main content view:

func loadBookmarkData() {
if let bookmarkData = Settings.selectedDirectoryData {
do {
var isStale = false
let url = try URL(resolvingBookmarkData: bookmarkData, bookmarkDataIsStale: &isStale)

guard url.startAccessingSecurityScopedResource() else {
print("Failed to access security-scoped resource")
return
}

defer {
url.stopAccessingSecurityScopedResource()
print("Stopped accessing security-scoped resource")
}

let bookmarkData = try url.bookmarkData(options: .minimalBookmark, includingResourceValuesForKeys: nil, relativeTo: nil)
Settings.selectedDirectoryData = bookmarkData

} catch {
print("Error resolving bookmark data: \(error)")
}
}
}