Currently, in the app, students are not able to see rubrics that have been attached to discussion boards unless they go through Grades > Submission & Rubric. This is not intuitive. A student would not think to look under GRADES to find a rubric. Instead, the rubric should be shown when a student clicks on the discussion.
When you go to a discussion in the web browser, you can click on the overflow menu to see the "Show Rubric" option. In the app, the overflow menu just shows "Add Bookmark".