How can late dates be earlier than the project planned start date?

Using P6 v19.12. I am working on a water treatment plant upgrade which has allowable work windows so the plant can remain in service. The contractor used calendars to achieve this. The contractor created new outage window calendars in an update, because the original windows were not used, and now there are activities which have late starts and finishes earlier than the planned start date. I thought late dates could not occur before the first day. What is causing this, and how can it be fixed?