Business Unit on assignment vs department hierarchy

Summary:

We have having an issue where worker assignment's business unit field is not matching business unit mapped in hierarchy based on departments.

We use the common set for all departments so users are not restricted, but we have found that people are not maintaining the user's assignment business unit properly. We can get the correct data based on their department and the hierarchy tree but looking for advice to enforce the business unit on the assignment to get updated properly.

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