You could try to go back with
"flatpak remote-modify --gpg-verify flathub"
Then enter
"flatpak update"
and restart gnome software center.
If this doesn't help you could try a solution found in this thread:
"sudo cp /var/lib/flatpak/repo/flathub.trustedkeys.gpg /var/lib/flatpak/repo/flathub.trustedkeys.gpg.bak"
"wget https://flathub.org/repo/flathub.gpg"
"sudo cp flathub.gpg /var/lib/flatpak/repo/flathub.trustedkeys.gpg"
"flatpak remote-modify --gpg-verify flathub"