Lost Task Reminders after Export/Import

I have helped a client move from Office 2016 to 365. The client relies heavily on Tasks, particularly the reminders.

We did an export to .pst on old system and an import on new system and everything seem to go ok. (for email, contacts etc). All the tasks got imported but none had the reminder box ticked. I tried importing into my Office 2019 (screen below) .. But same issue.

It is a car service centre (center?) and everytime a car comes in they want to remind the client of their annual service when it becomes due.

Is this an issue with Outlook or do I need to set something before the import?