I believe your problem is the same as mine. I found a solution that worked well for me.
I have a cube of 450 kilometers representing the atmosphere. When I tried to lower below a very low number, I also had a cutoff. The fog disappeared completely. My interpretation is that the floating point precision is too low. Therefore, Blender rounds down the number of 0.
The solution is to use the lowest precision where the fog is still rendered and to use a 3D texture to create variable empty spaces in a way that the final result is like creating particles in the volume. With a very low density and the right texture scale, the sum of the volumetric particles render make a continuous soft fog.
I wrote a complex shader with an altitude gradient. Here’s what I can share with you to develop the basic solution.
Here is the noise pattern with a ColorRamp to create a pattern with values from total black to total white. That allows you to have a custom group node with a slider parameter to vary from 0 to 1 easily.
Then, the trick is to use a math node with “Less Than” to control the volume percentage that will be tested to accumulate the volumetric samples. The lower-density factor comes from the fact that we don’t test (or add) all the steps within the volume.
The “Less Than” is your percentage to be tested.
The “Base fog value” here is “0.0001”. You can multiple it with a parameter if you want to increase the final result with denser fog.
You can also use an exponential value to control the altitude density based on the Z value of the generated texture position (0 to 1) of a “Texture Coordinate” node. In that case, you will multiple the “Less Than” value before the “Final density” shown here.
In other words, the only viable solution with very low values and large-scale atmospheres is to remove a part of the volumetric sum being added every step rate. That way, the final sum is still less.
Note 1: One problem that I had is the result of undesirable patterns. If you use a scale number that is too high, you can see different dense patterns forming, like being very dense on two edges or a dense cross being formed in the middle of the volume. At the moment, I have a pattern only visible when the camera is far away from above, but it seems correct within the volume.
Note 2: If your noise texture scale is something like “0.001”, your fog will look more like clouds.
I hope that helps.