With this simple setup, the metal caster to the bottom left stopped exporting via trucks when set to the yellow option (default) once conveyors (paused construction) were placed. Per the tooltip they should export via truck UNTIL belts have exported, then stop exporting via trucks.
Build: Update 4, v0.8.2a (b552)
This can be most directly be observed with sorting plants. The moment you connect anything to a port outputting a material, doesn't matter if built, unbuilt or construction paused, that material is no longer available for export by truck if the plant is set to export by truck if belts cannot. This reflected in the UI directly.
Either the tooltip should be updated or, preferably, the behavior should be altered to match the tooltip.