When I have "submit report PDFs by email" enabled why is the 'Email' button greyed out on the Completion Page of a Student(s) Lesson?

This scenario has 2 possible causes:
1: You've enabled your Course's Students can submit report PDFs by email from in the Lesson Report Settingsduring the Course of the Lesson this issue is presenting in.
If a Student has Committed the Lesson whilst submitting report PDFs was disabled, it causes the 'Email' button to be greyed out for that Student when enabling submit report PDFs:
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The Student can still Download a PDF of the Report and send it via their own Email Client if you enable Students can download a PDF of their report in Lesson Report Settings.
2: The Student has already sent the Report as an email by clicking the 'Send Email' button, the button will change to 'Email Sent', and be greyed-out - only 1 email can be sent per Lesson per Student.