Your databases could be corrupt. Have you ran recover on your database? You could just have mislinked records. If you delete records in the parent table without deleteing related records in the child table then your key fields could be crossed linked or your records could get mislinked by a design problems, such as not using serialized key fields.
Start with a cloned databases without records and run recover on your database to check for errors. If key fields are not serialized fields, I would update my design to use serialized key fields.
Sounds like you still have data in a related table and when you create the new record, an auto-entered serial number or other value used as a primary key is linking to data in that related table.