As I have no scene file here, I can only guess, which I always consider as not professional, as it takes a lot of time on both sides. I believe that you don’t like to share the original file, but a simlified version that shows that problem is always more welcome. Switch between Instances and no -instances, do you see a change in the Interactive Region Render?
Have you tried to place the particles in a Cloner Object, and go from there, materials on the child, not the Cloner Object itself?
If the scene file attached is not rendering as it should in both “render-worlds”, and if all the limitations listed from the Help-Content are not defining the problem, perhaps the Support needs to care about it.
[color: blue] Render Instances do however have a few drawbacks:
Render Instances are exact copies of the original geometry and cannot be deformed using Deformation objects.
Render Instances can only assume the current state of the original object and not different states of animation of the original object.
Render Instances should be made up of geometry because only geometry benefits from the corresponding advantages. Light sources, for example, will not work at all.
Objects that are a Child object of a Subdivision Surface object (i.e. serve as a source object for other objects) cannot be render instanced.
Sketch and Toon does not support Render Instances.
The most important differences between Render Instances and the original object:
Position, scale and rotation can be different.
They can have different materials (the original object must therefore have no material assigned to it).[/color]