Feature or bug, I discovered the same. I couldn't create tables with the default credential. 
To test, I assigned the default credential and a custom credential the same access rights to the storage container that is the target of the external location. When the external location was assigned to the the default credential, I got the error message PERMISSION_DENIED: The credential 'tekdata_databricks_dev' is a workspace default credential that is only allowed to

When I reassigned the external location to the custom credential, table creation was OK. I didn't find this behaviour in the documentation, but Copilot pulled it up from somewhere.