Ran into this exact problem a while back, the hint to the issue is in the tooltip for the Power node:
Interestingly, when I encountered this and was trying to troubleshoot the issue I discovered that if you use the HLSL pow() function inside of a custom node… it works correctly. I still have no idea why this is the case. Here’s your function with custom pow()'s instead of power nodes:
Thanks to @midgunner66 for originally pointing me in the right direction on this.