Texture atlas and particle system

I am using the particle system to display details on the ground. Think details of the terrain engine, without the terrain engine. So I build arrays of Particle() and set by hand size, color and position. I use UV Animation to store a 2x2 atlas. Back in 2.6 setting the energy of the Particle() would access the image p.energy = atlasLength - crop.atlasIndex) / (float)atlasLength;

It no longer works in 3.2, always the first image is the one displayed (upper left corner).

What is the workaround ?