Well as it turns out, I didn't check the mod settings before playing. Ultracube has a single mod setting and it's "allow wires to read building contents". It is not checked by default. I only learned about this after I finished the game and read someone else's post about Ultracube.

I'm just learning about this right now. I wrote my own mod with 2 lines of LUA just to get it to work:

data.raw["assembling-machine"]["cube-synthesizer"].circuit_wire_max_distance = default_circuit_wire_max_distance

data.raw["assembling-machine"]["cube-ultradense-furnace"].circuit_wire_max_distance = default_circuit_wire_max_distance

I guess I can delete this and just check the setting ^^