The issues are as follows: the Orion browser does not switch to dark mode and does not import bookmarks.
This happens on Fedora KDE 44, but not on Fedora Silverblue 43.
I know this because I contacted an Orion developer who advised me to reach out to the KDE community, saying: ‘It might be an upstream issue; it could be worth asking the KDE community what the usual expected behaviour is for system dark mode with Libadwaita applications running via Flatpak on KDE.’