Hi @Anonymous ,
It is recommended that you could create two Flows to achieve your needs.
One is used to create a new task, and the other is used to monitor whether the specified task has been completed.
Here you need to add a TaskId field in the List to record the task id in order to update the list item.
Delete When a task is completed and the following configuration and save Flow, then create another Flow as follows.
Use TaskId to determine if there is a specified item in the list, and if so, update the item.
Image reference:


Please take a try.
Best Regards,