Hi walterdp,
The data should be consist with planner. You can go to Planner, click Charts tab > Late to compare them. It's not related to Planner export feature or Excel I think.
I made a quick test and would like to share my finding. Seems for Incomplete tasks the column compares Due Date with export date/current date and shows no problem. But it converts to false when a task is Complete.
Seems the Late status only count the incomplete tasks that has a redmark which warns date expired in planner mainpage, for completed tasks the redmark disappeared automatically so the Late value will be changed from True to False.
It's a good idea to share this information in Planner UserVoice and the relevant team will keep improve our product. We appreciate your efforts.
Thanks,
Jim