Number 2, here we go 😝

The particle spawned by the tiaras are off by what looks to be about 2 blocks.

In the screenshots my character is positioned at the height where he would be stood on the top block of the tree and the particles are 2 blocks below that when they should be spawning at the players feet.

Looking here my initial guess would be that 1.8 has switched the entityLiving.posY to be the players feet position and not the head position, thus offsetting the Y position.

Instance:
minecraft-1.8.9
forge-11.15.0.1688

Mods:
Baubles-1.8.9-1.1.3.0
Botania.r1.8-248-MC1.8-a1aa50b
jei_1.8.9-2.14.0.67

Steps to Replicate:

1. Create a new world
2. Ensure JEI has Item Cheating set to true.
3. Give yourself a tiara (any is fine, I used Sylph and Ice Fairy).
4. Move around in third person view.

You'd be correct!