When a new student signs up sometimes the email gets lost in a spam filter and I manually activate the account from wp-admin > Users > Pending. For some reason after the student is activated I am unable to add the student to a course in Course > Admin. Previous students still show up, but something seems to be not finishing correctly during activation for them to appear in the AJAX autofill box.
Please check if the new user is shown in the wp-admin->users->all users.
If yes then please share the proper steps to replicate the issue and share the credentials to check this on your website.
PS: Please mark your reply as private while sharing the credentials.