Belts now need power. All of them. Including all the modded ones. Unmaintained.
Mods introducing new content into the game.
Augmented or new ways of transporting materials - belts, inserters, pipes!
If the power is cut or removed the direction of the underground belts switch, disconnecting them, and they don't switch back on power return.
I'll look into that. I'm currently making powered versions of vanilla belts, I'll relase the bugfix when I release that. Destroying and then placing them again should fix that for now.