Are these real students? Enrolled in course with the student role?Not you using 'switch role to'? What are the dates for the assignment? What type of assignment is it - file upload/ online? Nothing needed to submit?
These are not real students. We created a manual account to test it. Available date is April 8th and due date is set as April 15. The type of assignment is file submission.
When you go to the course > Participants and look at Roles, does the test student have the student role? And just checking this is in an actual course and not on the front page.