Tested on Unity 2019.3.0b12 on Editor & Standalone Win
Still repro on Unity Editor 2020.1.0b1 width latest HDRP/staging

/!\ Only happens when using Vulkan Graphic API

To reproduce :
1. Open test project and Test scene
2. Make sure you are using Vulkan graphic API
3. Make sure everything is green on the HDRP wizard
4. Make sure you have "Sky and Fog" Profile asset " in the "Sky and Fog Volume" gameobject
5. Make sure you have "Scene PostProcess" Profile asset " in the "Sky and Fog Volume" gameobject
2. Select the sphere and its material
3. Add the texture called "grey" on the detail map

At this point the rendering should be all black (both on Scene & Game view)
.
You can minimize this effect on the game view by enabling the "Stop NaNs" checkbox on the camera to restore part of the view but the planar reflection should this be pitch black.