Cant turn in molten core quest once maps are available.

Didn't turn in the hammer to Oswald in Ziggurat Encampment before going into maps - this zone is now inaccessible after finishing act3 cruel.

Is this a known feature or a bug im unsure, but would think the quest would be closed if the zone was inaccessible.

Anyone else had this issue and found a fix or is it just dead?