Google assigns a unique internal ID to each user you create. When you delete and re-create a user account, events that the original user was invited to are not tied to the new user account.
Because the new user has a different Google identifier, they do not receive calendar notifications when the organizer modifies the original calendar event.
Resolution
Avoid deleting and recreating users.
If you re-create an account and its username had existing events, uninviting and reinviting the re-created user does not correct the event. Start over with a new event and invite the new user.
