Game mode: [Online]
When drinking all of the water in a water-filled glass flask it creates an empty glass flask. When you try to fill that new glass-flask it disappears. This only happens when you are carrying a stack in your inventory and your bar (1-8) is full.
edit: Actually this seems to happen with any amount. When i had a single flask it seemed like it was working but if you try to move the flask in your inventory then it will also disappear.
Steps on how to reproduce issue:
1.Fill your action bar (1-8)
2.Have a stack of water-filled glass flask
3.Drink all of the water
4.Try to refill the flask
If you have an empty slot on your action bar it will show up there. If your bar is full it will disappear. When it does show up on the action bar and you try to drag it to the stack in your inventory, it will disappear as well.