How to reproduce:
1. Open the attached project named "U - BUGREPORT.zip"
2. Open the "Bugreport scene"
3. Enter Play mode and observe the Game window

Expected results: Distant Volumetric fog is rendered in grey color
Actual results: Distant Volumetric Fog is rendered in solid black, forming a dark line on the horizon

Reproducible with: 2020.3.13f1(10.5.1), 2021.1.14f1(11.0.0), 2021.2.0b3(12.0.0), 2022.1.0a2(12.0.0)
Could not test with: 2018.4.35f1(Path Tracing was not available), 2019.4.29f1(HDRP 7.7.0 - Volumetric Fog does not seem to be rendered)

Notes:
1. If Path Tracing or Volumetric Fog is disabled distant horizon fog is colored in grey.
2. If the Camera height is below the Fog's Maximum Height parameter value distant horizon fog is colored in grey.