@Dr-Sassi Thank you for your thoughts.
It's probably not practical to upload the scene file as my particle cache is 11GB? Let me try trimming the frame range which would make it a much smaller file.
I did create a new scene with a much smaller particle cache file from RealFlow and everything works fine, just as expected. Same structure and setup as my larger scene. So my process must be correct? I'm wondering if it's a processing time-out thing? My main scene has 1.5M particles so maybe RS doesn't wait long enough for the frame of particles to load and populate the matrix? Perhaps I should try Standard Renderer just to see if it is a rendering breakdown.
You mentioned the MoGraph Cache tag. In my smaller scene, I was able to cache the matrix and delete the particle cache and everything stuck and worked fine. In my larger scene, if I try to cache the matrix, it turns green when done, but then when I click to any frame, the tag turns red and the matrix nodes lose all data. I even tried baking the cache to files to save on the memory?
I would appreciate any other thought you may have at this time. Otherwise, I'll let you know when I have a sample of the scene file uploaded for you to check out.