I think this can be due to the use of Generic adapter for Dremio as a Data source as the execution engine would not have been able to adapt and create an execution plan based on the capabilities of Dremio.
Without the VQL, it is difficult to know why you are experiencing this behavior. However, for this scenario , I will do the following checks,
1. Ensure if Dremio supports Outer join.
2. Analyze Execution trace to see why it is not been delegated.
If you are a valid Support user of Denodo, I will suggest you raise a case describing the issue along with the whole VQL of the view and the Dremio data source VQL, as that would be helpful to understand the background better, reproduce the scenario and address the issue.
Hope this helps!