Steps to reproduce:

1) Create course as admin, add teacher to course

2) As admin, go to Enrolment methods > Manual enrolment > Notify before enrolment expires. Set this to "Enroller and enrolled user". Set Notification threshold as "1 days"

3) Login to teacher account, add a student to course.

4) Edit student's enrolment end date so that enrolment ends in little over 24 hours.

The bug:

Notification message should be sent at 6 AM next day in our environment. However, only the teacher gets this message. I have reproduced this in both 3.9.1. and 3.5.13.