I wanted to lay out a few learning I had from this thread that were't explicitly stated but I found through testing.
I found helpful was to ensure the SP List is the source for task item creation in planner. This allows a flow that creates the Planner Task when a SharePoint Task is created. This can be achieved using a simple flow to, but one helpful tip is to capture the ID of the Planner Task in the create flow to ensure matching can be done later. Important Tip: Create a few column in the SP List, 1. 'PlannerTaskID' so you have a place to store the ID variable and optional add a 'Planner' status in case you would like delineation between SP Task Status and Planner Task Status.
Step 1-3:Trigger Task creation from new SP List Item, store Task ID Variable, and pass SP Item variables to Task
Step 4: Pull ID from Task into variable
Step 5: Update SP Item with Planner Task ID
Now that you have a flow that creates planner task, and the Task ID exists on both the Planner Task and the SP Item, you can create a new flow to subsequently update the SP item when the Task is completed in Planner:
Steps 1-2: Trigger upon task completion, query the matching SP List
Step 3a: Conditional filtering of target to match the TaskID
Step 3b: Apply changes to SP Item
Bonus Tip: If you use MS Project 2016 (Not the Online Version) and the SP List is a "Task" List you can use the Sync function with SP to additional drive updates bidirectionally from Planner to SP to Project (I realize this a fragile setup but it works! 😁)
