Sorry, I don't know. I would suspect something in flatpak OCI backend is not working right. Maybe wait and see if the RHEL flatpak update improves things? Or maybe RHEL flatpak needs some OCI patches backported? This update installs just fine on Fedora.
@sumomu, "Can't pull from untrusted non-gpg verified remote" sounds like an issue with flatpak, not this update
As for the openh264 error, the error is completely unrelated to this update. It's trying to say that the openh264 update from flathub (not from fedora) requires a newer flatpak version than what's available from centos. This should already be fixed as part of https://bugzilla.redhat.com/show_bug.cgi?id=1754450 in RHEL 7.7.