This is caused by how the status of a task is calculated, and in particular the days late. Below is a snippet from the documentation of how a late status for a task is determined, and if so, how many days the task is late by.
Displays the number of days late considering the following:
- Whether or not a baseline exists
- Project/task progress
If a baseline exists, then days late is calculated as project/task finish date minus baseline finish date. A positive number indicates that the project/task is late according to the baseline. A negative number indicates that it is early according to the baseline.
If a baseline exists and the project/task finish date is the same as the baseline finish date, then the project/task is considered late if it is not completed and it was due before today’s date. In this case, days late is calculated as today’s date minus project/task finish date.
If there is no baseline, then days late is calculated if the project/task is not completed and it was due before today’s date. In this case, days late is calculated as today’s date minus project/task finish date.
In this scenario, the tasks are being determined as late because they have a Finish Date which is after the Baseline Finish Date. This is by design according to the first clause, which applies because there is a baseline for the project. Thus, the days late value is being calculated by the formula
Days Late = (Task Finish Date - Task Baseline Finish Date)
With the above, the Days Late value will be positive, meaning the task is late according to the baseline finish date, causing it to display as late in the Gantt Chart and the Task List page.