It is not a 30 min lock down, and it's always been there. The way it works is that from the time you get the mission you have a 45 min timer. So if you finish in 18 minutes, then 45-18 leaves 27 minutes or around 30 minutes. If you finish in 44 minutes then ther would be only 1 minutes left in the timer. Monster missions work in the same way. The timer starts when you get the mission. If you fail or quit the mission then this timer is erased and you don't have to wait, if you succeed it stays.
There is separate timer like 2-5 minutes for the quest giver to give missions after quiting nothing to do with this one, just to prevent too many instances being created and the server crashing.