Repro:
1) Open the project in the attached EnvLightTest.zip and load up TestScene.
2) Bake light and observe that environment light is contributing to the generated lightmap.
3) In Lighting > Environment, switch Static Lighting Sky to "None" and rebake. Observe that the generated lightmap is now black.
4) Switch Static Lighting Sky back to "HDRISky" and rebake. Observe that the lightmap is now unexpectedly still black.

Expected: Rebaking with Static Lighting Sky set to "HDRISky" and "None" should generate non-black and black lightmaps respectively.
Actual: Except for the first bake, the lightmaps are always black even when Static Lighting Sky = HDRISky. See attached envbake-hdrp9.gif.

This only happens when Visual Environment -> Ambient Mode is set to "Dynamic" in the associated Volume Profile. To my understanding, Ambient Mode only affects the ambient probe and should not affect lightmap baking.

It is unclear whether this is due to an issue with HDRP or with lightmap baking.

Verified using Unity 2020.2.0b5 and HDRP 9.0.0-preview.54. HDRP 10.0.0-preview.27 also exhibits this behavior but HDRP 8.1.0 does not. This suggests the issue was introduced somewhere between HDRP 8.1 and HDRP 9.0.