If you do the same limted time quest within a period of time betwixt Japanese midnight It wont give you a Voucher the second time. For me i am eastern, it is possible to do the early morning one before jp midnight and again after jp midnight if it is still in my log and before the mid day limited time quest pops up and get a voucher from both. But if that same queest pops up in the later night limited time frame it wont count, as i already did it since JP midnight.
But as for the quest disappearing within the 2 hour period they should be available that is weird.