Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.51
Broken: version: 2.91.0, branch: master, commit date: 2020-11-25 08:34, hash: rB0f45cab862b8
Worked: (newest version of Blender that worked as expected)
Short description of error
I've noticed for a while now that if I bring in a vdb file, to a scene that already has fog via a large cube with principled volume shader applied, once I either import the vdb or, if it's hidden and I unhide it, the scene fog via the large cube will suddenly turn off. There's no way to get it back other than to reload the scene/restart Blender.
Exact steps for others to reproduce the error
Make a scene, envelop it in a large cube, apply principled volume shader. Something like 0.025 density or thereabouts. Whatever is thick enough to see the scene and appreciate the fog is there.
Import a vdb file and apply a volume shader to it. The main fog should disappear.
Then try putting the fog on the cube up to 1.0 density and you'll notice you get fog back, but, it's not as dense as it should be, even at 1.0 density.