Right clicking a stack of items always picks up half of the items. The server side does not agree with that in a Brewing Stand! This is an issue both in singleplayer and multiplayer.

How to recreate:
1. place down a brewing stand and put a stack of nether wart in the top slot.
2. right click the stack to pick half of it up, then left click on a slot in your inventory to drop it there.
3. to reveal the bug, do one of the following:

• Exit the Brewing Stand GUI and enter it again.
• Exit the Brewing Stand GUI and enter any other GUI.
• Click the stack of 32 nether wart in the Brewing Stand.
• Click the stack of 32 nether wart in your inventory.
• Relog.
• Die.