iacdxb,
just click with the right mouse button ON a material in the material browser.
Then select "Render All Material".
I have the same problem like you and this little trick helps. Its not perfect, but I am sure the corona guys will come up with a bug fix.
Logged
It's not the end of the world. But you can see it from here.
C4D R20 Studio on Win10, AMD 7950X, RTX4090
C4D R20 Studio on macOs 10.13.6 on iMac 27" i7 End 2015