from MS documentation it is unclear what works and not with Exchange Hybrid
I can see a lot of "may" but no definitive answer and what the variables are
- If your organization is not using Exchange Online for your account, you may not be able to comment on tasks in Planner. link
- Planner has full functionality when your user has a product license that includes Exchange Online. Planner users without Exchange Online may have issues with viewing or adding comments to a task. link
- email will only be sent to users who have a product license that includes Exchange Online. Users at organizations using on-premises Exchange Server or hybrid configurations may not receive all Planner emails. link
