Celluloid uses portals, simply forcing use of portals will do the job. Just add this in your apps part
apps:
celluloid:
environment:
GDK_DEBUG: portals
Because celluloid is still using gtk_file_chooser_native as the given code above shows, this issue is persistent. When they move to the new gtk_file_dialog even this will also not be necessary.
This will work in any averagely modern distro like Ubuntu 22.04 LTS. Same thing is done with gnome-text-editor also.