This is a hard one to explain... I searched for it being reported before, I didn't see it anywhere. When you grab a potion arrow from your creative inventory, then click on a different potion arrow (While still having the other potion arrow in your cursor) it stacks the already existing arrow in your cursor.
Hope I explained that correctly, sorry if it was already reported, like I said, it was kinda hard to explain.
Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-82986. You can find or add further information on that report.
Please search before reporting any bugs, as it's likely that one exists already.