Steps:
1a. Open new 3D template
1b. Add HDRP in package manager (10.0.0-preview.27)
1c. select Fix All and Create One on HDRP wizard popups
2. select the current renderpipelineasset by double clicking current HDRP asset in Project Settings > Graphics
3. in inspector, check the cache settings of the asset at the bottom (expected to be greyed out since VT is off)
4. toggle Virtual Texturing support = on in project settings > player settings > other settings
5. restart (or save and restart)
6. check HDRP asset

Expected: VT on = HDRP asset VT cache settings are editable, VT off = HDRP asset VT cache settings are greyed out
Actual: VT on = HDRP asset cache settings are greyed out, can't edit

Repro: 2020.2.0a20 + HDRP 10.0.0-preview.27
No repro: 2020.2.0a15 + HDRP 10.0.0-preview.1, HDRP 8.2.0

Notes:
- MacOS only
- In 2020.2.0a19 may see slightly different actual result: VT off = error when selected HDRP asset and broken UI in VT cache settings in inspector

Workaround: add or remove a "scripting define symbol" in Edit > Project Settings... > Player > Other Settings > Scripting define symbols, then click "Apply" - this will trigger a reload and ensure the correct state for Virtual Texturing support. You can use any text for the "scripting define symbol" just to trigger the refresh.