A Lookup Field Has the Value "Unknown record"

This issue occurs when a long-running delete transaction is in progress, and a child record is updated while its parent record is being deleted.

1. Create a custom object as a parent object.

2. Create a custom object as a child object.

3. In the child object, create a custom lookup field that points to the parent object.

4. When creating the child/parent relationship, in the section What to do if the lookup record is deleted? select Clear the value of this field. This sets the child lookup field value to null if the parent record is deleted.

5. Start a long-running transaction in which the parent record is deleted.

6. While the delete transaction is running, update the child record.

As a result, the child lookup field contains the value Unknown record. This value displays in the UI and in the query editor.