I'm not sure if there is an "Action" which can do the same, but you'll be able to retrieve more information with the action: Send an HTTP request"
URI: https://graph.microsoft.com/v1.0/me/mailFolders/Inbox/messages/[Message Id]
[Message Id] -> Dynamic value used from the trigger -> retrieves only that message
The send HTTP gives you a bunch of information including the due date of the flagged status.
You'll be able to use the action "Parse JSON" to retrieve that data.
(Edit 25-11-2022): Under "Flagged status" there you will find 2x the same name: "DateTime" & "Timezone". Use the one which corresponds with the Duedate (view the body of the HTTP request to see if it's the first or the second)
You'll be able to insert this date in the due date of the planner card.