Manager is unable to add additional participant reviewer to some of his employee performance doc

Summary:

As a manager, I want to 'manage additional reviewers' (aka: 'manage participant feedback') to these 2 workers below. For Stanley, after I clicked the Manage Additional Reviewers, the add button does not display. Refer to third screenshot below.

Why is it for Sharma, the add button is displaying after i clicked Managed Additional Reviewers?

But for STanley, I do not see the ADD button?

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