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The discount is applied to the Microsoft Learn profile for the email address used to register for the event. If you didn't have a Learn profile associated with the email address at the time then the voucher couldn't be added
This is a community forum not support, try Microsoft Credentials support https://aka.ms/credentials-support
PS MCIDs no longer exist https://learn.microsoft.com/training/support/faq?pivots=profile&source=docs#what-happened-to-my-mcid