Well
here is a forum thread from folks who actually play the game which appears related.
It is either a software problem, or the GPU simply isn't being utilized very much. Usually when you have performance issues and one or the other is at 100% then it would be a "bottleneck" of some description but with that sort of high-end hardware it is pretty certain to be a software problem.
As I'm sure you know a "bottleneck", when it comes to CPU/GPU is when there is widely disparate performance characteristics. By way of comparison, many years ago I had a 350Mhz K6-2 CPU paired with a Radeon 9250. The CPU was a massive "bottleneck" and this meant that games that the GPU would be otherwise capable of running were out of the question for me. Conversely, putting an old ATI Rage Pro into a Pentium 4 would "bottleneck" in the other direction.
"Bottleneck" behaviour that only appears in specific software is typically a result of the software, not the hardware. A poorly optimized or programmed game/software could very easily underutilize one or the other and peg the other at full utilization, making it appear that poor performance is a result of the hardware when it is a characteristic of the software.
This is particularly the case given that your CPU is one of the absolute latest available. There is no way a 7600 is going to bottleneck a 1070- older Haswell CPUs can easily keep up with a 1070.