In Mantis 1.0.0rc1:
Go to manage->Manage Configuration->Workflow Thresholds
Any changes to "Status to set auto-assigned issues to" wont´t be accpted. this value is always on "assigned".
Saving "Status to set auto-assigned issues to" don
Moderators: Developer, Contributor
You should report this in the tracker at http://bugs.mantisbt.org/ .