If you get this quest and the NPC is not showing in Chromie time Legion Dalaran for you to turn in you will have to go to your faction capital and see Chromie to return to the present. Then you can return to Dalaran and turn in the quest to the NPC who will now show, then return to your faction capital, turn Legion Chromie time back on, and proceed from there.
At the time of this comment, upon returning to Legion Chromie time the NPC has disappeared again, so my assumption is that this may need to be repeated every time you receive an herbalism quest from gathering.
The quickest way to do this will be to set your regular hearth to your faction inn in Dalaran so that you can port back and forth without waiting for either it or your Dal hearthstone to come off cooldown.