The cause for this is actually the long list of lens shaders that gets added by the "Aerial Pespective" feature built into Sun & Sky.
For now there is a workaround.
Proceed as follows:
- Create a Sun & Sky system as normal, by creating a daylight system and choosing "mr Sun" and "mr Sky", affirming the pop up question of "do you want to ad a 'mr Physical Sky' environment" with a resounding YES.
- Then, change the skylight option from "mr Sky" to "Skylight"
- Change the Skylights option to "Use Scene Environment"
The difference is that now one needs to do any "tweaks" to the sky strictly in the "mr Physical Sky" shader, and to do this, one needs to "drag-and-drop" it (as an "instance") from the "Environment" dialog into a free slot of the material editor, then tweak away.