Question regarding filter:marked as late

[Mattias]

I have a strange thing in the filtered view of tasks when I apply the filter Late tasks.

One summary task is marked as late as seen in one of the screen shots attached.

The task that creates this "late date", is a milestone marked as 100% complete in the project.

The only thing I can see that could be the cause of this is the constraint task-20 days, but I do find it strange that it should create a late warning since its complete?