With the latest Moodle update(2.3.4), when teacher is previewing the quiz and quiz has been set to submit automatically open attempts, then it does'nt do that when time is up.
If you enter with(or as) a student account then it does submit when time is up.
So probably something is wrong with the quiz preview mode for teacher.

Steps to reproduce:
1) Create a quiz
2) Set a timelimit about 20-30 seconds.
3) When time expires set Open attempts are submitted automatically.
4) Add couple of questions to quiz
5) Preview the quiz as teacher or admin
6) Quiz should be submitted automatically when time is up.

I tried to reproduce this at demo.moodle.org but there is still 2.3.2 running. This happens with 2.3.4.