In Toad v10.5.0.41, the 'Configuring grantees' feature under the Roles tab is not setting to default. This is not a problem with v9.7.2.5. Also, this happens for SYS schema too. So, it can't be rights related.
WORKAROUND:
1. Click on 'Show SQLL' and then 'Send to Editor'
2. In the Editor, add the line
DEFAULT ROLE "<ROLE_NAME>";
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STATUS:
Waiting for fix in a future release of Toad for Oracle.
If you look at the SQL generated, it is missing the correct user in the statement