It seems to be fine now. I suspect whatever was generating that releases.json file was cached or not updated by the time the website was generated.

How does this generation process works? Does it listed to fedmsg messages?

It looks like its using fedmg.
In looking over how things are built, we are using this script https://pagure.io/fedora-websites/blob/master/f/getfedora.org/build/releases-json.py.

So my assumption that the site got built prior to the data being available is likely the scenario that occurred here.

How is the build triggered? The script doesn't look like a service that triggers rebuild job when new release is out.