Course welcome message sent from course contact with surname that comes first in alphabetical order

As reported in the discussion Re: welcome email shows old admin as sender, the course welcome message sent from the course contact with surname that comes first in alphabetical order.

Steps to reproduce:

1. Check in the site administration that the course contact is set to the default value of teacher.
2. Create a course and assign several users the role of teacher in it, making sure that the first user who is assigned the role of teacher does NOT have a surname that comes first in alphabetical order.
3. Enable self enrolment and set 'Send course welcome message' to 'From the course contact'.
4. Log in as an ordinary user and self enrol in the course.
5. Look at the course welcome message email.

Expected result:

The course welcome message email is sent from the course contact who is first assigned the role of teacher. This behaviour is described in the help text "If sent from the course contact (by default the teacher), and more than one user has this role, the email is sent from the first user to be assigned the role."

Actual result:

The course welcome message email is sent from the course contact with surname that comes first in alphabetical order. This is not good because there is no way of changing it.