Since the "fix" that made comparators able to recognize full stacks of items in the brewing stand (MC-7058), the comparator can no longer recognize the difference between a brewing stand with a brewing ingredient in it and one without. Obviously this ability to recognize if there is an ingredient in the brewing stand is essential to pretty much any redstone work that includes brewing stands and comparators...

How to recreate:

1. Put 3 water bottles in a brewing stand.
2. Place a comparator next to the brewing stand, facing away from the brewing stand, and extend a line of redstone from the output of the comparator.
   • The output will be 11.
3. Place a single brewing ingredient in the top slot of the stand.
   • The output will still be 11.
4. Continue adding more of the brewing ingredient to the top slot until the output increases.
   • The output will not increase until you have 10 ingredients in the stand.