When I use a plane (or in this case a cylinder) to generate reflections on the building, these artifacts appear
It’s a simple physical material for the wall and a photo for the reflection map.
I’ve been working this way since more than a decade with C4D and later with Corona, but for some reason, in the latest Corona versions it doesn’t work anymore.
I am sorry to disappoint you, but this has always worked like this in Corona. You can read about this under “black highlights” here: https://support.chaos.com/hc/en-us/articles/4528311915025-Known-bugs-and-limitations
Yes, but it happens also without using a “light” material.
Hi there, can you share your scene with us? If so, please use the ‘‘save project with assets’’ options from C4D and then send us a zipped file. The link to share it can be found in my signature below. I also believe it might be a black highlights issue. 
