Just to confirm, the users have a primary job role on their profile correct? If they do, you might want to contact support. I have used Resource Planner since its Beta (and the old Flash-based Resource Grid) and I've never seen a role disappear from a task just because a user assigned themselves.
Now you might run into the issue where their primary role is not the same as the original role and their primary role is not in your filter. That I have seen.
And for the hours cut in half, again, is there only one task assignee to the task? If you have multiple people, the hours are split evenly unless you use the simple duration type and manually update the hours. (Like back my agency, we'd have tasks that were like 20 hours for the art director and copywriter, but then 2 hours for the Creative Director as they were just assisting/mentoring.)
Hope that helps. If you are continuing to have issues, I'd recommend reaching out to Support. They can then let you know if it is a bug or point out what you might be missing.