RHEL 9 Download actually downloads RHEL 8.8 beta
When you go to add a new box, and choose "Download an operating system", there's an option for "Red Hat Enterprise Linux 9.0 x86_64".
However, if you look at the file it is downloading, as of today, it is downloading the RHEL 8.8 beta instead.
Looking at the code, it appears that in https://gitlab.gnome.org/GNOME/gnome-boxes/-/blob/main/src/assistant/rhel-download-dialog.vala it downloads https://developers.redhat.com/download-manager/rest/featured/file/rhel?tag= + user agent
However, if we follow that URL (with the handy tool, curl) we can see it is redirecting to the RHEL 8.8 beta ISO:
$ curl -Ls -o /dev/null -w "%{url_effective}\n" 'https://developers.redhat.com/download-manager/rest/featured/file/rhel'
https://developers.redhat.com/content-gateway/file/rhel/8.8.0_Beta/rhel-8.8-beta-x86_64-dvd.iso?cookieCheck=de998a63-d633-49e2-956e-0c922856cfe8
I suspect that the URL that is being used probably worked when the RHEL 9.0 ISO was the latest ISO released (or featured), but that probably should point to something more specific for RHEL 9.0 if you want to hard-code a list of installers.