I did a few more attempts. It turns that if one waits for every sequence of text to be played out instead of clicking them away and proceeding fast through the mission, the button does appear at the end. Perhaps there's a hidden timer somewhere before the button unlocks?
Unlike in Beta 1, in Beta 2 the first tutorial mission cannot be completed. The very last action of the first mission is to activate the 'all ahead full' option. however, this button remains greyed out and cannot be clicked.