Hey 
I sometimes run into this issue, where objects turn transparent in rendered view. Until now I never knew what the issue might be, but it is extremely annoying when I notice it too late and it really affects my work.
A snap from the original File:
The test file:
Now I had this effect once I toggled the “cast shadows” checkbox in the properties panel.
Enabeling that option again brings it back to the normal state.
I copied this object out of the original File where it affects most objects there. Unfortunately I cannot upload the original file due to confidentaility reasons. When creating a new geometry in that file it sometimes shows the same effect, sometimes not… Rebuilding sometimes works, sometimes it doesn´t…
TransparencyIssue.3dm (180.1 KB)
I am pretty sure this also happened to objects in files where i was not toggeling this option at all.
I attached the file, I hope this also shows when someone else opens it.
Thanks in advance for any help for that issue.