Running into an issue with specifically a brand new DEP MAC (we're a small mac shop, 6 machines total), and I've got jamf connect set up to skip the account creation and create a hidden temp admin account. After the user logs in with their AAD creds it then prompts them for the local account password (which they don't have and doesn't exist), and they can't get any further. Is this a specific JAMF connect thing, or a Big Sur thing? Seems to have worked previously on DEP machines without an issue.
Is JC set to enable Filevault too? And do you mean the user logs in with their Active directory credentials? Not sure the entire setup and why you are deploying a temp hidden account, but the end users account should be the first account being created (when they sign in) subsequently granting a secure token and enabling Filevault easily. You can deploy a admin account via JAMF separately as well as demote their account from admin to standard via policy too if that was the reason you didn't want them signing in first.