When using a dropper to dispense water bottles into a brewing stand with its contents previously removed by a hopper won't work unless water bottles are placed inside then removed manually by the player.
How to duplicate:
1. Place a brewing stand with 3 bottles inside, then place a hopper underneath it.
(optional: remove the hopper after it removes the bottles)
2. Put a dropper facing the brewing stand and put bottles inside of it then activate.
MC-111753 Hopper/Dropper will not input water bottles into brewing stand after potions have been removed by a hopper from brewing stand