When you have a 1:N relationship and set the Remove Linkcascade option for the Delete under Relationship Behavior, the link information in the lookup of the child record still exists after you delete the parent record. When you click the link in the child record, you receive an error that the parent record does not exist.
The link information will be deleted by the Deletion Service, it is not removed real time.
Wait until Deletion Service runs. By default, it runs every 24 hours. The Deletion Service will remove the link information.