I'm seeing the same thing on iOS 27.0.1 in a React Native / Expo app (react-native-screens native stack). The back button goes blank and stops responding after a few visits to a screen whose header changes width between visits, so the system collapses and re-expands the back title. I never saw this on iOS 26. Hiding the back title (chevron only, headerBackButtonDisplayMode: 'minimal') avoids it for me. I haven't found another workaround.