A Veeva user X can be added as a team member in an Event to have a visibility on this event (X can see the event in the calendar and can manage participants if he has been granted for).
Once the user X is deactivated, a trigger deletes these sharing records. Upon reactivation however the records do not get recreated since being an Event Team Member is a customized process. We have to remove and add again the user X to the concerned event.
As we can have deactivation because of a long period of inactivity (holidays, disease...) the suggestion is not to delete the sharing records after a deactivation.