ACR doesn't default from the time DFF is made mandatory

Summary:

We have a rule to default a DFF based on business unit selection. And It was working perfectly till the time the DFF was not mandatory. But as per business need we needed to make the DFF mandatory but post this rule has stopped to default the field.

When we select BU, it just gets highlighted to RED and doesn't default.

What could be the problem. Is there any limitations that ACR doesn't default to a mandatory fields?

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