Adrian is right that these “disco” artifacts (love that description btw) are due to resolution of the irradiance volume. You can see one spot of this appearing in my scene as well:

The higher in resolution the more detailed the light bake but the longer it takes and more data is saved to your hard drive. The lower you go in resolution the smoother the bake (no disco artifacts) but you lose detail. This is the balance game we have to work within.
I found that the disco artifacts are much less noticeable in the final scene once materials and fog are present.