For others who may be experiencing this bug:
We've found a feasible (if improbable) work-around: Move the teacher's courses to higher in the list of categories.
The bug occurs if the teacher's course(s) don't appear in the first 250 returned by the query. The query returns courses in the order they appear in the course categories.
Our teacher's courses are (were) in Marketing -- quite a way down the alphabetical-by-subject category list. By moving her courses to a category above the subject list, she's able to find her courses in the import file selector. (Most of our teachers use backup/restore. This one teacher has requirements which necessitate the use of import. If you have many teachers using import, this work-around will be a non-starter.)
Kudos to my not-a-programmer Boss for figuring this one out.
(We're running M2.2.1 -- one tiny upgrade shy of the version where this bug is fixed, and no prospect of upgrading until the end of the semester, 3.5 months hence.)