Unable to fetch reference records in Workbench - Workaround
By default, in Workbench "Allows SOQL Parent Relationship Queries" will be unchecked. So, check the "Allows SOQL Parent Relationship Queries" checkbox to fetch reference records in Workbench.
1. Go to "Settings".
2. Check the "Allows SOQL Parent Relationship Queries" checkbox.
3. Click "Apply Settings".