OS, Java version, ect is irrelevant for this bug. In-game, you can fill a potion stand by facing a dropper into the stand and dispensing potion bottles into the stand, then emptying the stand with a hopper. The bug occurs after having dispensed exactly 3 potions (in any combination) to the stand from a dropper, and having those potions be removed by either a hopper or a mine cart hopper. After exactly 3 potions have entered and exited the system, the dropper will no longer dispense potions into the stand, despite it being full. Solution to fixing the problem manually is to break the stand, thus, the problem lies with the potion stand, and a value attached to the stand is not being subtracted as the hopper takes the potions.