I had a similar issue again today morning.
Render was at 4k, Multi-Pass only 3-4 masks.
The .psd file was saved but the masks were missing. Only beauty-pass was saved inside of the psd file.
So I checked VFB and Picture Viewer to see if they were rendered or not (I thought I just forgot to activate all my masks)
In VFB were all masks visible (so they were actually rendered) but not in C4D Picture Viewer. Non of them were visible there.
A bit later, I restarted C4D and rendered the same scene, same settings, and everything worked as expected.
I also tried to reproduce it and played around but was not successful.
Not a problem at all, just in case someone else get this odd behavior too :)
C4D R20
RC 4
Edit: Just on small thing:
Corona Compositing tag on a Corona Sky object doesn't work properly.
- Create Corona Compositing Tag on a Corona Sky
- Enable Object Buffer
- Set up Multipass
- Rendered mask is pure black.