The learning MFE now has a preview mode for learning content that is under the /preview/course... set of URLs. Both the sidebar and the breadcrumb view link to non preview urls.

eg. The link to the parent section is like
http://localhost:2000/course/course-v1:test+test+test/block-v1:test+test+test+type@sequential+block@bc0d339f753747d9b9fa162f59111673
instead of like
http://localhost:2000/preview/course/course-v1:test+test+test/block-v1:test+test+test+type@sequential+block@bc0d339f753747d9b9fa162f59111673

Note: This is not a new bug, this was true in the old preview as well.