I'm seeing the same as I work my way through a series of tutorials. I have to assume its a bug in SwiftUI?

Your fix makes group a CNGroup instead of a CNMutableGroup, although why that should help I am not sure

No, they just closed my bug, and I felt unable to release on catalyst :( Recently I see they have updated the documentation for this call https://developer.apple.com/documentation/messageui/mfmailcomposeviewcontroller/1616887-setbccrecipients with a note to say it doesn't work under Catalyst. Annoyingly it ALSO doesn't work when running the iOS app on Apple Silicon. Which I see as a much more serious problem. I believe being unable to put contacts into Bcc compromises email address privacy, and they really should fix this, rather than just documenting it. Please report this as a bug to them- as the more developers that do this, the more likely it will get fixed.

This article may help https://blog.uptech.team/how-to-build-resizing-image-in-navigation-bar-with-large-title-8ba2e8bcb840