Backend event registration sends email with numeric From address
When performing a backend event registration for a contact, the default From address for the confirmation email is the primary email of the user. Sending the email using this address results in the contact receiving the email with a From address which is simply an integer. The integer is the value of the id field of the expected email address in the civicrm_email table.
If instead a defined CiviMail From address is selected, then the contact receives the email with that From address as expected.