Right-clicking on a stack of items generally splits it in half; but on the input slot of a Brewing Stand, it picks up the entire stack.
Also, if I have some of an item in the stand and some in my inventory, shift-clicking the stack in my inventory does not add it to the stack in the stand, which seems like it could be related (and is unlike the behavior in e.g. a furnace's fuel slot).
Hi, this is not a duplicate. Unlike the linked bug, I can split other stacks normally, and this bug has been present since brewing stands were introduced.