I encoured this problem, i don’t know what causes this.
I have only a simple glossy node, with 0.2 glossy.
When i render, ( oh sorry my bad english, i can’t describe it ) - see this image: the very black parts.-what are these?
I added 6 emitters from XYZ axis, i added Emission to floor too, but the problem still occurs.
I did a fast low-quality render:
EDIT: Here is the wire: