Unarchiving a child record when the parent record is missing or unavailable will fail, unless the toggle is switched on for Enable Unarchive When Parent Archived. This will resolve the issue in some cases, but will create orphaned child records. This feature is located in the Settings tab, under Unarchive.
Scenarios: Record A is the parent of Record B.
Usecase #1 Parent Record A is archived.
Child Record B is archived.
Parent Record A is unarchived and in the process receives a new ID (C).
Child Record B can be unarchived.
Archive maintains the relationship of Record A to Record C. No feature toggle is required.
Usecase #2 Parent Record A is archived.
Child Record B cannot be unarchived. The child record cannot be unarchived when the parent record (A) is still archived.
Usecase #3
Toggle on Enable Unarchive When Parent Archived from the Unarchive Settings tab.
Parent Record A is archived.
Usecase #4 Parent Record A is archived
Parent Record C is (re)archived.
Child Record B cannot be unarchived. The parent-child connection was lost when the Parent Record was rearchived.
Even unarchiving Record C (which in the process receives a new ID - Record D) will not resolve the issue.
Toggling Enable Unarchive When Parent Archived from the Unarchive Settings tab will not allow for the unarchiving of the child record in this case.