You could always set up the C-8 as a new hub, then once you get the UI go to Settings > Backup and Restore and find it there. Unless you are already there? Then maybe you need to do a soft reset to get back to the "new hub" migration UI.
I've done the 2 approaches above @jtp10181 . The thing is both the wizard or the "normal" restore page don't show the "migration" backup type... I really think this is a bug to list the migration created backups since they are not from "cloud" type anymore.
Is there any chance you have an underscore in your email address? Or some other non-alphanumeric character in your email address? I seem to recall that was an issue for some users. It may have been corrected already though... Tagging @bobbyD from the Hubitat Support Team.
I think they fixed the _ issue a few versions ago.
On my C8, if I go to Backup and Restore, then select "all" to see all backups I can see my old backup from the C-7 and have a button to migrate from another hub. This is what the instructions say it should look like for the migration backup as well.
Hmm... its a mystery then. Only other thing I could think of is to go on the old hub and force another migration backup. Unless you have already tried that as well. Support was tagged up above so they will jump in when they see this. @support_team