@dgelessus @ccc The code that I tried for the ButtonItems were pretty much the same as you suggested. After a lot of trying I think it's due to the NavigationView generated by the UI editor. If I create the view programmatically it works. I changed the code in my app to use a pyui file for the general frame. In the frame there is a placeholder where I insert the manually created NavigationView. As subview I insert another view loaded from another pyui file. It's definitely not straight forward anymore but at least I made some progress after being stuck for two days with the NavigationView that didn't work. By the way: this also fixes the trouble with the wrongly centered buttons.
@omz Could this be a general problem that the NavigationView generated by the UI editor has some flaw in it making it "misbehave"?