According to the developers, it seems that the event is working as intended.
That means that here is something not being completed correctly.
It sounds like you must clear Abyss layer 1 to get credit, not any of the others, so this could be the issue. Users are stating that they can clear the dungeon simply by killing the boss.
For now, I would recommend taking a SS of your game guide before it resets, and send in a ticket
if you miss out on the prize so we can find out exactly what was missing.