Volumetric fog has a heavy temporal filter in order to make the quality/performance acceptable for realtime. You can adjust the temporal weight using the cvar: r.VolumetricFog.HistoryWeight
It defaults to 0.9, lowering it should make the fog more responsive to lighting, but it will be less stable (more noise/flickering).
A better solution most of the time is to just try to achieve your scattering effect some other way (most games just use camera facing billboards), there’s a good chance it will look better in most cases while also performing better.