We're seeing this too and I can't figure out how to disable it. It's quite confusing and very poor design if this is really how it works, with no ability to change the behavior. It seems like any grading should obey the gradebookroles setting but Forum does not.
We are preparing documentation to explain the new functionalities of the forum among the faculty of our University and this is really confusing. We have also tried to find out if there is a setting or an option to choose which roles should be graded in the forum grader, but unable to find anything.