Resolution: Works As Intended
Affects Version/s: Minecraft 1.9.1 Pre-Release 3
Fix Version/s: None
Please read carefully: I am not complaining that Dragon's Breath leaves behind a glass bottle. I understand that that's WAI. However, hopper behavior is inconsistent with these bottles. For every other ingredient, more than one can be pushed into the ingredient slot (i.e. you can push up to a full 64 stack of Nether wart in). However, although glass bottles stack up to 64 as well, you can't push glass bottles into the ingredient slot, even when there's already a glass bottle there. This is the only item which exhibits this behavior; all others that can be pushed into that slot of a brewing stand can also stack as normal. That makes it unique and unintuitive behavior (unexpected behavior for new players), which is why I get the feeling it's unintentional and a bug.
1) Put a hopper facing down into a brewing stand.
2) Put a stack of any brewing ingredient in the hopper--the full stack will transfer to the brewing stand.
3) Replace the ingredient slot with Dragon's Breath and brew a lingering potion; leave the glass bottle in the slot.
4) Put a glass bottle (or a stack of multiple ones) into the hopper--it will not be transferred to the brewing stand, even though bottles are stackable and there's only one bottle already in the destination slot.