Not sure if this is a setup, but I am not sure about the logic of display via 'View posts by user' option.

As an admin account I changed settings under under 'forumng_monitorroles'

The roles 'Student', 'Contributing student' and 'Full student' were all ticked.

As a tutor account I went to a 'No groups' forum and selected 'View posts by user' at the bottom of the discussion table.

Users with the roles of 'Contributing student' or 'Full student' on the course
appear in the list of users.

However, users given only the role 'Student' don't appear, even though this box
is ticked under forumng_monitorroles.

The correct behaviour is as follows for a non-groups forum:

• List all users who have a role on the forum, course, course category, [any
  parent course categories if nested], or site level, that is one of the roles on
  the 'forumng_monitorroles' list.

In other words, if you ticked Student in the list, then all students on the
course should display when you click View posts by user. So it is a bug that
the students do not appear.