Unable to pre-select file when opening a local source in Nautilus from inside a Flatpak
This is about the feature added in commit 67fbd531. It doesn't work when running inside Flatpak's sandbox.
Applications running inside Flatpak's sandbox are unable to see the list of installed applications on the host machine. Therefore, it's not possible to get to a GAppInfo as is possible when running unrestricted. Specifically, the g_file_query_default_handler call fails. One can expect that the subsequent call to g_app_info_create_from_commandline would have also failed.