About shadows and complex 3D layers
When using shadows with layers such as particle systems, text objects, or replicators that are set to 3D, shadows are cast from one part of the object onto another part. This is called self-shadowing.
Modifying the object in a way that causes rasterization prevents shadows from displaying.
In some cases you may be able to find another way to perform the effect that does not require rasterization. For example, in the following images, rather than modifying the opacity of the particle emitter, which causes rasterization, you can modify the opacity of the particle cells and maintain the shadows.
In the second figure above, rasterization interferes with the depth order of the particle system, and particles appear in front of the letter A. In the third figure, no rasterization occurs, and particles appear in their proper depth order, in front of and behind the letter A. For more information about how rasterization affects shadows, see Shadows and rasterization.