Flatpak 1.15.7 is released 3 weeks ago, but not yet enter most distro yet, not even flathub build bot has it yet.
fcitx5-unstable's fcitx is updated to support the new flag --socket=inherits-wayland-socket, but you still need flatpak 1.15.7 on host to support it.
So far, arch linux already has 1.15.7 and should work again with flatpak version.