Le sigh. So I went back to test Ghostship's wonderful materials and lights by setting up the background node the way he explains in his tutorial.

I did the setup, exited the material room, it looked great. Then I entered the mat room again to adjust a setting on my model's eye texture. Surprise! When I moved back to the Pose Room the background node I set up was disconnected and the original default setup was auto-plugged in back.

This is very bizarre. I retested twice. This happened after a Windows 10 update so no idea what got messed up. Will file a bug report regardless. Just wanted to see if anyone manages to replicate.