I know as a Moodle admin, I do not get forum email unless I assign myself as a teacher in the course. That may be true of the manager role too.
Effect of no teacher in course
Where are you giving them the course creator role? in the entire site, in a category or in the course itself? That will make a significant difference in answering this question.
I'm not giving them the course creator role, because I don't want them creating courses. They would just be managers within a specific course, which gives them additional privileges which the teacher role does not have. I'm just concerned that there may be functionality that is programmed specifically to look for the teacher role.