Try this. There's a cube in the scene with fog density set to 1.0 so you can't see anything. Look in the scene collections for the vdb file which is currently hidden. Unhide it and the fog will reduce in density visibly but will still be at 1.0

If I open the file you've provided I see no difference in material preview or rendered mode with or without the vdb object being visible. Please answer the questions from my last comment and provide precise instructions how to reproduce the problem with the file you've provided.

I uploaded a video to show how it looks for me. Unfortunately the audio didn't capture but if you just re-read what I said above it's about as precise as it needs to be. In the video I open the file, note that there's nothing visible in the scene because the fog density of the shader (that's applied to a cube encompassing the scene) is set to 1.0. I unhide the vdb file and it erroneously causes the fog density to change, so now, once the vdb file is unhidden, we CAN see the scene. However, the density has not changed from 1.0. At the end of the video I re-load the file and it's still broken (the volumetric fog is neither 0 or 1, it's somewhere in between). You have to restart Blender to fix it. Using Eevee.

I can't reproduce this in Blender 2.91 or the current master branch.

Watch the attached video. Open the new Blender file and do what I do in the video. I've attached the vdb file. Essentially import the vdb cloud file, add the default volumetric shader and then scale it up and down and watch the density of the large cube's volumetric shader go in and out in proportion to the vdb file. It's a scaling issue of some kind related to some aspect of certain vdb files.