If you see an Online Course in a learner's User Interface (UI) that has an Expiration (or other) Date that wasn't set up through the course's Availability settings and you also see that they are enrolled in a Curriculum that includes this Online Course, it is likely that the expiration date has been inherited from the Curriculum. When a learner enrolls in a Curriculum, any Online Courses that are part of this Curriculum will inherit the Due Dates, Expiration Dates, and Access Dates from the Curriculum.
For example, if you have a Curriculum that has an Expiration Date of September 1, 2017, all learners enrolled in this Curriculum will display this Expiration Date listed for all the Online Courses within it.
If a learner is enrolled in a single Online Course and then, at a later date, they enroll in a Curriculum that includes this Online Course, the existing Due Dates, Expiration Dates, or Access Dates will then be overwritten. The displayed dates of the Online Course will appear to have changed because the date is now being inherited from the Curriculum.
Note: If the Online Course belongs to a Course Bundle the expiration date setup on the Online Course will not be respected and therefore no expiration date will be applied to the User's enrollment.