Thanks for the scene, Andreas, yes, that works here as you have described it. I had all Image passes on, which didn’t include RGBA nor Object Buffer. Then, no RGBA pass is written. I tried many combinations, but R19 writes the Background file as file. R18 did not with the same set up, you are correct.
Please contact the support:
https://www.maxon.net/en-us/support/how-can-we-help/
It shouldn’t be saved out, if not explicitly directed to, as it would perhaps overwrite an existing file (Yes there is a warning, but we all know how that works close to a deadline.). For a separate buffer rendering, one might have changed something in the scene or render settings even, to get a wanted result.
Sorry, I can’t do more here.
My best wishes