Hi Patrick,

I would check in your administration block - Site administration - plugins - enrollments - program.

The enrollment duration block may have a value other than 0 which would cause the learner to become unenrolled a period of time.

If this is only appearing in a single course then something may have happened with your program enrollment method in the course (such as disabled). From the course administration block - Users - Enrollment methods. You should see program listed as an enrollment method.

Other enrollment methods can also affect program enrollments so you may want to look at any of the settings associated to the methods used in the course.

Hope that helps.

Peter