This works as expected through the interface, however it is possible to circumvent the limitations in the interface by manually entering an URL.
1. Login as a teacher and start a discussion in a single simple discussion forum.
2. Enter the URL of the forum in order to create a new discussion (e.g. MOODLE/mod/forum/post.php?forum=n).
3. You'll get the interface and be able to add a new discussion topic. This also worked for both a teacher AND a student.
Afterwards if you view the forum and have debugging turned on you'll get an error thrown because get_record() will get multiple results:
Error: mdb->get_record() found more than one record!
line 1302 of /lib/dml/moodle_database.php: call to debugging()
line 1262 of /lib/dml/moodle_database.php: call to moodle_database->get_record_sql()
line 1242 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()
line 144 of /mod/forum/view.php: call to moodle_database->get_record()
This does mean you'll never see more than one discussion topic regardless.