Found it! As usual, it's not a bug...it's a feature. To be exact, it's this (from the change log of 1.18): "The 3ds- and obj-loaders now load transparency information."
Your sky has a transparency value of 100 for whatever reason. The loader now loads this information. That, combined with the way how transparency works in the software renderer, cuts the light intensity in half on the sky. Just add a
sky.setTransparency(-1);
and it should look better.