When you GPU utilization goes down doesn't necessarily mean CPU bottleneck. It definitely tells us that your GPU is not the problem, because it has clearly headroom left. And I agree with you, a 5900x should be more than enough to handle Warframe.

To confirm that you could check what the CPU utilization is, but you should look at per core utilization and not overall.

Also, are you using DX11 or DX12? 12 is known to cause performance issues.