Currently when a planned completion date is later than the actual completion date, the demand allocation holds on to the planned completion date. The problem this causes is showing demand allocation to resources on work that is already complete. This may show a resource over allocated when in fact they have availability.
To avoid 'not real' allocations, we've asked our PMs to update their planned completion dates on tasks that are completed prior to the planned completion date. As you can imagine, they don't want to do this for a couple reasons 1) they have a lot of tasks in their projects and it would take time to do this and 2) they lose visibility to work that is getting done sooner than planned.
Ideally when a task is marked complete, the allocations past the date the task is marked complete would not allow for any more demand allocation on that task. Complete means complete and no more work will be done.