When a splash potion is thrown, its effect particles (swirls, x shapes, etc.) appear at a point offset from where the potion landed originally, usually at the nearest corner between 4 blocks. This has been tested multiple times in Minecraft 1.11.2. Screenshots are provided.
Steps to reproduce:
1. Throw a splash potion of any type.
2. Observe where the bottle lands and where the particles originate from. If it isn't reproduced, throw it in another place, preferably in the middle of a block.