When the first letter of the first name and the first letter of the last name are selected in Participants to display all students meeting those criteria, these values are not reset upon exiting the screen or the course.
So... if you enter the Participants screen for another course the same parameters will be in effect,
but...if no students match these criteria none will be display
and... the ability to change the paramters will also not be displayed, so students actually in the course are hidden.
This behavior seems to occur until logging out or you reach a course that actually is able to display a student so you can change the parameters.