I am not sure if my issue is related, but it very well could be. In my case we are using a UISplitViewController and in one of the views, the right one, the title bar is not displayed initially. But if I push and pop something to the therein contain UINavigationController, it appears again. This is tested on Beta 5 of macOS Sequoia.

I would assume that depends on whether the bundle identifier is the same and whether it's uploaded to the same app slot in App Store Connect. If yes, users would just seamlessly update into your new Catalyst app.