Hi @RanjeetSingh ,
I think the most likely explanation is that the payload for https://jhu.instructure.com/api/v1/courses/1188/content_migrations is a blank array because the course has no content migrations. The URL you mentioned - https://jhu.instructure.com/courses/1188/pages/000-migration-exception-log - is not where Canvas would display content migrations normally. This URL indicates some user created a page in the course showing some migration exception log, most likely copied & pasted. Course 1188's own content migrations would be at https://jhu.instructure.com/courses/1188/content_migrations.
I tested the API with a course of my own that did contain content migrations, and I'm getting the same info in the GUI & API. I hope this helps.
Sam