So your computer accounts are AAD only but User accounts Hybrid? .
Have you tried with FQDN, like ADDOMAIN.local\admin ?
Can you ping your AD domain, DNS resolution works?
Actually SCCM console delegation to workstation is a pain, with CM you need to add computer accunt as well to CM server local admins or so, so this specific need probably would not work. But you should access network shares with AAD machines still.