The bug

If multiple translucent particles coincide when using the Fabulous! graphics setting, they appear to combine to form a brighter texture than otherwise.

How to reproduce

1. Create a repeating command block executing the following:

   particle block light_blue_stained_glass ~ ~4 ~ 0 0 0 0 1000

2. Build a room made of bright looking blocks around the fountain of particles
3. Compare visuals for Fancy and Fabulous! graphics