When enrolling students manually with a default enrollment duration set, the enrollment end date is not set for users unless the "Enrollment options" menu is expanded in the enrollment pop-up.
Steps to replicate:
-Create a course with an enrollment method of manual.
-Edit the manual enrollment method for the course and enable "default enrollment duration" and set it to 90 days (or whatever time period makes you happy). Save Changes.
-Go to the Enroll Users screen and hit the "Enrol users" button. Search for a user and then hit "Enrol" and "Finish Enrolling Users".
**User will have a start date, but no end date for enrollment.
-Now hit the "Enrol users" button again.
-Search for a user and this time open the "Enrollment Options" menu at the top, then hit "Enrol" next to the user and then "Finish Enrolling Users".
**This second user will have a begin date and an end date for enrollment.