Why did a common task I already performed and set to completed not show as completed in every instance of the task across multiple implementation projects?

You must set the task status individually for each task in each implementation project. The setup data for a task might need a different set of data for a different implementation.

The status of a task doesn't update based on its status in a previous implementation. For example, you implement Financial Control and Reporting and complete the currency setup. Later, when you implement Procurement, you want a few additional currencies for paying suppliers but not for financial reporting.