As I said, openh264 doesn’t come from Flathub’s servers. It’s distributed directly from Cisco, just as it is on Fedora, for example. There have been many reports of connectivity issues, but there’s not much anyone can do about it.
But as bbhtt mentioned, there are some apps that could benefit from h.264 support that aren’t set up to use ffmpeg-full. For those apps, openh264 is the only available option. The only example I know of is GNOME Web, but I’m sure there are others.