Update: chevron-only ('minimal') stops the blank button but wasn't enough alone. The full fix also needed header options set once per mount (static headerTitle/headerRight, no setOptions on updates) and react-native-screens 4.17.1; on 4.16 the chevron had drawing artifacts. No help: fixed header widths, solid header, hiding bar-item glass, UIDesignRequiresCompatibility. The iOS 27.1 simulator can't reproduce it; it draws the back button outside the nav bar.