Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Dear Locke,
The Planner Premium's data is stored in Microsoft Dataverse which is Project for the web not the normal Planner which is stored in Azure. So you can't edit the data in My Tasks directly and not all fields will appear in My Tasks in Planner which is an Azure location. You need to go to Microsoft Dataverse to edit the plan data and see all fields by clicking on Go to premium plan.
The Pinned lost shows an embedded UI of Project for the web in the Microsoft Dataverse location, so you can edit the data and see all fields there directly.
For your reference: Frequently asked questions about Microsoft Planner - Microsoft Support
Microsoft Planner for admins - Microsoft Planner | Microsoft Learn
If you create a normal plan which uses the standard template, the plan is stored in Azure and you should see the checklists in My Tasks and Pinned.
Hope that the information above helps you.
Thank you for your time and effort.
Sincerely
Cliff | Microsoft Community Moderator