C4DtoA 2.6.1 [4186e70b]
Arnold core 5.4.0.0
New scene.
Add a floor and a box for reference.
Add an Arnold sky. Set it to Physical sky.
Now turn on the IPR and drag the Physical sky's resolution parameter arrows down so that the value decrements to ~500. Once you have scrolled from 1000 (the default) to 500 the lighting should at some point suddenly change from bright to dark.
This has to be a bug?
Increasing the sun size param acts as a threshold to prevent this weird effect from happening. Turning the physical sky resolution down further shows that the scene reverts from dark to light multiple times at random decrements. The resolution values at which the lighting flips seem to be different for different project scenes.