Why Don't Tasks Assigned To Me in Microsoft Planner Don't Show Up in To Do?

Hi JBLT-77​ 

This usually comes down to what kind of plan the second one is, and whether you’re a member of its group. I can’t see your setup, so here’s what I’d check, in order:

1. Plan type. Premium Planner plans (the ones with goals, sprints and so on) don’t always behave like basic plans. Other users report assigned tasks from a Premium plan not showing up in To Do, while tasks from a standard plan do. If the plan that works is a basic one and the other is Premium, that is the most likely explanation. 
2. Group membership. Microsoft’s feature comparison says Premium plan assignments only show in “Assigned to me” and To Do when the assignee is also a member of the Microsoft 365 group tied to the plan. Being named on a task isn’t enough. Check that you’re actually listed as a member of that second group, not just assigned tasks. 
3. To Do settings. In To Do, go to Settings, turn “Assigned to me” on under Smart lists, and make sure Planner is on under Connected apps. 
4. Same account and organization. Make sure the second group belongs to the same organization as the account you use in To Do. A plan from another tenant, or one where you’re a guest, may not sync.
5. Test it. Sign out of both apps and back in, then create a brand new task assigned to yourself in the second plan and see whether it appears. If tasks show in Planner’s “Assigned to me” view but never reach To Do, it’s almost certainly the plan type or the membership issue above.

If you tell whether the second plan is a basic or Premium plan, I can narrow it down further. Hope that helps!