Vulkan PRIME fail on dual NVIDIA gpu system

Hello All.
On my dual NVIDIA GPU system I can successfully use PRIME on OpenGL applications.
However, Vulkan has problems.

System Summary:
OS: Gentoo
DE: LXQT
GPUs: NVIDIA GeForce GT 1030 (this drives the monitors), NVIDIA GeForce GTX 1660 TI (this is for Prime use)

nvidia-bug-report.log.gz (1.0 MB)

When trying to run vkcube an error message is created.

$ vkcube
Selected GPU 0: NVIDIA GeForce GTX 1660 Ti, type: 2
Could not find both graphics and present queues

Although this is the GPU I would like to be used, it is not successful.

Running this same command for the GT 1030 is completely successful.

$ vkcube --gpu_number 1
Selected GPU 1: NVIDIA GeForce GT 1030, type: 2

I have attempted both of these command variations with and without the __NV_PRIME_RENDER_OFFLOAD=1 environment variable and got the same results.
Testing OpenGL using glxgears works perfectly for both cards. Which I verified using watch nvidia-smi on a separate terminal. So it appears to be some conflict between Vulkan and PRIME.

I realize that a two NVIDIA gpu system is rare, but I am completely lost on how to diagnose this failure.