When using the Flatpak version of Digikam, the global menus aren't enabled and not exported to the top panel. No problem if using the Debian package or the AppImage version.
digiKam team do no maintain the Flatpak bundle. Please report this problem as DOWNSTREAM bug to FlatHub team. https://github.com/flathub/org.kde.digikam/issues
https://github.com/flathub/org.kde.digikam/issues/60