Simplify my previous reply: My custom backbutton by default hides system back button. When it do toggleHidden() to hide the custom one, isHidden=true. iOS 26 and priors: the system back button is never appeared; iOS 27PB: system back button appears right after the custom one is hidden -> allow users do unexpected back behavior. If it's intentional behavior change, i'll add self.navigationItem.hidesBackButton = true, if not i don't need to make change for up comming iOS 27 release.

Your hint saved my life. I need to clean two Build Settings in my test target to No Instrument Program Flow & Generate Legacy Test Coverage Files