When a Network adaptor, IP address or a network related record (seen in the below tables) is marked as absent, there are cleanup jobs that run and delete all these records. The problem here is that it when the Table Cleanup runs and delete CIs, it doesn't delete the relationship record and leaves orphaned Relationships.
1./ Run discovery for one of your servers. Discovery will create a relationship record between the server and the network adaptor.
2./ Change the adaptor/IP on the server and rediscover it
3./ Discovery will create a new adaptor/IP record, a new relationship record and will mark the old one as absent.
4./ When the cleanup job runs (OOB, it deletes records that are absent for a month but it is costumizable), it will delete the record but the relationship will not be deleted
Enable Cascade Delete by ticking the check box on the 6 table cleanup job records mentioned. You can find them in the below link:
Please replace <instance_name> with your instance.
NOTE: if the above link does not show 6 table cleanup jobs then please search for the missing one by searching for the table names provided above.