It looks like you're using the old Flatpak from Flathub (0.0.8). Fluxer now provides its own official Flatpak, which you can install from the download page:
https://fluxer.app/download#desktop
The Flatpak install command listed there is:
flatpak install https://pkgs.fluxer.com/flatpak/fluxer.flatpakref
Might be worth trying the current official Flatpak first and checking if the issue still occurs there.
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Comment by @SolunaCG