When using Boost or any other theme based on it, this patch should fix the issue:

.path-mod-assign [data-region="grade-panel"] .mform .fitem.has-popout .felement {
   width: 100%;
   overflow: auto;
   height: calc(100% - 4rem);
}

1. In a course (with manager or teacher role) turn editing on and add an activity of the type assignment.
2. In the Grade section of the settings set the Grading method to Rubric. Save and display.
3. In the Actions menu of the activiy, select Define Rubric.
4. Set a name and fill the new rubric with at least 4 criterions with some long level descriptions. Save rubric and make it ready.
5. Enrol one student user to the course so you can grade the activity.
6. When grading the assignment zoom in the rubric.
7. Check the rubric table (criteria, levels) is in the pop-up area.