I am new to Unity, but I assume it is a bug? Problem affects all Gradients in VFX Graph (which you create yourself etc), not only this particular one.
All color pickers in Gradient types in VFX Graph look like usual RGB pickers (without intensity slider etc)
It seems like Unity opens Gradient Editor instead of HDR Gradient Editor.
Same color picker in the same exact Gradient looks like this in 2021.2

