I'm seeing the opposite, where server groups and internal db is available in creating a WLAN (individual servers are not, by design). Where are the RADIUS server groups configured in the hierarchy? Specific to RADIUS servers, there's no priority, the groups added should just be one where multiple RADIUS servers are prioritized in the group.
Can you attach some screenshots to include where the server groups are configured in the heirarchy relative to where the WLANs are created? if you're seeing just RADIUS servers, where are those created in the heirarchy?