Why isn't the Inbox header always showing the submitter name?

When I am in the inbox of a form, why is it that sometimes, the name of the submitter shows and other times, it says "Submission 65?"

That shows when the field you've used as header is empty. If you check and confirm if such field is required on your form or shows data on the submission itself, but still does not reflect on the submission header on the inbox page, let us know. 

Here is how to check the selected field as header on the inbox page:

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