Configuration.
https://wiki.archlinux.org/title/Flatpak
Given you have multiple portals installed … maybe you need to configure that?
~/.config/portals.conf
[preferred]
# Use xdg-desktop-portal-gtk for every portal interface...
default=gtk
# Or fall back to xapp
org.freedesktop.impl.portal.Settings=gtk;xapp;
Also, it may make sense to check on the services;
systemctl --user status xdg-desktop-portal xdg-desktop-portal-gtk