The Participants list filters has only All and Students as options. If you have any other custom defined student roles, it will not show them as Students in listing. 

It should show all possible graded roles. 

To replicate:

• Clone the standard student role, call it student2 "Student Two"
• Give it the same settings in the Allow role assignments, Allow role overrides and Switche roles tabs as the standard Student role
• Make it gradable in Site admin > Grades >  General settings > Graded roles
• Create a new course and enrol a test user in it as standard Student role and another test2 user in it in the Student Two role.
• Verify the filter works in the Enrolled users screen by changing to filter both Student and Student Two
• Go to the Participants screen and verify both are there in the Current role > All participants filter of the list.
• Choose Current role > Student and verify you see only the student in the default student role and not the student in the Student Two role

Desired outcome:

The pulldown filter list here should show ALL students here and not just those in the default role.

Probably the best criterion would be to test for Graded role. This is what the grade book does and so it properly includes all graded roles here. 

Another alternative would be to have the pull down work just like the Enrolled users screen filter does, which is show all the other roles that have users enrolled in them in the course already.

Tested and can replicate this with Clean and Boost in 3.2.3 - .5 and 3.3.2.

This is VERY IMPORTANT for sites that have external enrollment systems such that the Teachers are not allowed to see or change the Enrolled users screen but are limited to the Participants screen for their course roster. It should really have the same features as Enrolled users but in a read only fashion.