This seems related: Problems with the materials preview - rhino wip 6 for mac
I’m worried there is a new bug here, but I’m not seeing it on my end. As I mention in the above linked topic, we used to have this bug, but we fixed the materials themselves…however, we never automatically overwrite (or delete) the contents of Render Content once they are written.
Please bug me if you are still seeing this after trying the steps in the above linked topic. My guess is that these users tried an earlier version of Rhino 6 for Mac and (buggy) default Render Contents are still hanging around.