So even though it was in Submission, it wasn't available
I can confirm this is expected behaviour at the moment. The reasoning for this behaviour was that teachers would set the submission deadline and switch the phase manually only after that (e.g. after checking that all students have their work submitted).
In a future version, these things should be more flexible (and easier to set up) as every student will be controlled by his/her own workflow. So it will be possible to have individual deadlines without the need to change the phase and/or block others from their workshop tasks.