If a user has no right to see a project (no Browse Project permission) than she/he shouldn't be be choosed in the assignable users list (in the 'assign issue screen').

If we set the "Assignable User" permission to some project roles (for example: Developers and some custom ones) and we put some users&user groups to these roles, but some of these users don't have the permission to see the project (no Browse Projects permission for them)... we still can choose these users in the assignable user list in the 'assign issue screen'.