>>106485801
Is XnView MP also a Flatpak? Maybe it's a bug with how it's doing things in the Flatpak environment.
I think you would have to use flatpak-spawn --host
like this but it requires granting extra permissions to the Flatpak:
$ flatpak-spawn --host -- gtk-launch org.kde.krita
Portal call failed: org.freedesktop.DBus.Error.ServiceUnknown
Hint: --host only works when the Flatpak is allowed to talk to org.freedesktop.Flatpak
Why GIMP worked but Krita didn't I'm not sure.