I have noticed this issue after installing Corona 4 Release version, when i create simple metal (or mirror-like) material with ior 23-999 and glossiness between 0.7-0.99, rendering models with material applied becomes weird and slow, its happens both for IR and Production rendering in all complex scenes no matter of lighting, with hdri or simple Coronalights, caustics on and off, with new lightsolver on/off, the only thing that helped me is a fresh scene with one sphere in it) then it is rendered well.
I attach screenshots of comparison between the 5th and 25th pass
I want to upload GIF but it is too big)
3ds max 2018
Windows 10
Xeon E5-2683 v3
32 GB RAM