easy rendering for Rhino in Windows
Open the first slice in the image editor.
Add the other slices (omit the one you opened.)
Save Tonemapped Image As... and select an alpha channel format.
I recommend keeping the complete nXtImage as well, if you have the space for it. It contains a lot more data than a .png.
I did a quick test here and did not find the same problem. So I would like to try your model if I could. Can you send me the model and the HDRI file you are using? firstname.lastname@example.org.
Also, what version of the farm clients are you using. If you right click on the farm in your system tray, select restore. Then go to About in the farm.
I will try it on my machine.