Hi,
I already tried to repro it, but I can't.
Repro steps:
1. New scene with a teapot of your choice
2. Create Corona Sun with "shadowing from clouds" activated
3. Create Corona sky map with cloud amount=1
4. Create Corona sky map with cloud amount=0
5. Create Corona select map
6. Plug sky from (3) in slot 0 of the select map
7. Plug sky from (4) in slot 1 of the select map
8. Plug the select map into (any used) environment slot
9. Render and switch skies using the select map
-> when slot 1 is selected in the switch map (the sky using no clouds), you get cloud shadows from slot 0.
Good Luck