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Regarding the planner task resource, could you please change the priority of the task in the planner UI and try to fetch the details as mentioned below and let us know if this is working as expected.
Try changing the priority of the task in the planner to urgent.
Try fetching the details of the task using Graph API where priority remains as "1".
Also, in the documentation https://learn.microsoft.com/en-us/graph/api/plannertask-update?view=graph-rest-1.0&tabs=http its mentioned as only the percent completed property can be updated.
After verifying the above-mentioned details, if you are still encountering this error, I would recommend you to raise a support case with Microsoft Graph, a Support Engineer will be able to look into this issue and assist you better. You can raise support ticket from https://admin.microsoft.com/#/support/requests
Hope this helps.
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