The output connection doesnt actually need to be built. If you build the input side, plan but not build the output side, build the pipes/conveyors hooked to the output side the goods are transported through the underground connection. Output side doesnt need to be built.

The please us discord was from the time I didn't get any notifications on this forum.
Now that is solved, it's a feasible way of communication.

On your report, I don't quite understand the configuration that is wrongly transporting goods.
Is it : input belt -> underground transport -> underground transport -> not connected ?

Input belt (built) -> underground transport (built) -> underground transport (not built only plan, construction is paused) -> output belt (built)

With this configuration material will be transported through the underground system despite the second underground transport not being built.

Should be fixed with version 5.2.0 which is released here.
Thanks for the report