Weird Volume Artefacts in Viewport and Render

Hi, I have a complex scene, with several volumes representing smoke and clouds.
The scene is split into several files, and the main file just contains a camera, world lighting, and links to collections from the other files instanced into the scene.
So the volumes are each in their own collections and linked/instanced.
The smoke volumes themselves were each created by running smoke simulations then copying one of the .vdb files from the resulting noise cache into my assets folder and importing it with Add > Volume > Import OpenVDB.

The clouds were created with the “mesh to volume” modifier. I’ve never seen a problem with the clouds.

I’m long done editing those volumes.

My problem is that completely at random, when I open the main file, sometimes I see something like this:

the artefacts are persistent (I can rotate around them) and can occur for any of the volumes.

Case for this being completely random, after capturing the above screenshot, and while writing this, I quit blender without saving, re-started and re-opened the same file from the splash menu, and got a different result:

I could live with this, but sometimes the artefacts will bleed into the render, sometimes producing a “wall of smoke” around the actual volume, sometimes unwanted shadows and sometimes just black blotches. There is no correlation between the volume artefacts in the viewport and the mis-behaving volumes in the render either.

Example artefacts in the render:

(“shadow” on the roof)

(black patch)

The only thing I can think of is that maybe the .vdb files from the noise cache are somehow not compatible with blender’s volumes, but that doesn’t explain the random behaviour.

Platform details:
Blender 2.91
Renderer: Cycles
Apple Macbook Pro 15 inch 2018
MacOS Mojave 10.14.6

I think I can answer my own question now, it did seem to be the .vdb files from the noise cache, I swapped them for the equivalent files from the data cache and the problem went away, however it’s not ideal as I only get the lower-resolution noiseless volumes.

Had the problem now too, even if the post is older,
Just in case it causes problems again.
In my case, it was the number of Max Transparency Bounces.