Though "retain materials" option is marked beta I would like to inform you about the following:
I created small scene in Rhino (cube and sphere) and assigned materials (img 1).
Then I added one object (cone), saved the file, pressed open in KeyShot, checked "retain materials" and the materials shifted to the next created in Rhino object or disappeared (img 2).
This thing is really very incovinient. Half of time I spend reassigning materials after any change or addition of objects.
Please, fix the bug. It will save a lot of time for those who use Rhino!