it kind of seems like you are in the same boat as me and no one has responded yet. I did find a site that has a work around but it is a dirty way to go about it. if you are upgrading from CentOS 7 to Alma8 you can try the following.
add your proxy setting to
/etc/leapp/files/leapp_upgrade_repositories.repo
but then also add it to
/var/lib/leapp/el8userspace/etc/dnf/dnf.conf
once you update these files you can use the bash script mention in the link below. it will require a little bit of modification. like the part about the SSL check and the proxy should match your info and update el= to 78
http://neilrieck.net/docs/linux_notes_leeap.html#script
but the process is to open 2 sessions to your host. one will run the script, the second will run the preupgrade and upgrade.
hope this helps.