You have created an Org Structure (either manually or using Migration) and again, you have used Migration to add some more Org Units. The Migration is successful but the Org Units are missing in the Planning Area of the Organizational Structure.
After successful migration, those Org Units will be in Planning Area (inactive) and it needs to be activated later. There are two possibilites that may end up in this issue. They are,
You have deleted the Org Units in the Planning Area.
You have performed the rollback in the Planning Area which is delete all inactive changes and revert the Org Structure to the last active version.
The system works as expected.
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