I had the Potion of life from the Jade Figurine quest in my stash. I acquired the potion in NM.
I made it to hell and kept the NM potion in my stash. Eventually I completed the Jade figurine quest in hell while still having the potion of life in my stash.
When I completed the quest, I still had the NM potion of life in my stash, and did not have a Hell potion of life in my inventory.
Now the quest is complete and I do not have the potion, I am short 20 life…
Just guessing but maybe it’s unique and the game registered that you already had one? Or maybe it dropped on the ground? Not sure why did you keep one in the stash though.
It happens. I was just too busy doing other things, being rushed, doing runs, etc. By the time I even thought about using it, I was in hell and just put it in my stash for later use.
Its an item like mephisto’s soul stone, you can only have 1 at a time, regardless of the difficulty, but where do i get my potion now… you only get it when you complete the quest and its already completed.
Do I have to make a new din now? =/
That sucks. Don’t know if you can get one if the quest is completed. 
make a new din now and use those potions immediatly. It takes less than 1 second to do this and it makes no sense to store it