Condensation on just 1 duct in unfinished basement
If the ducts do not have sealed air spaces at top and bottom when the ducts get cold the air around it becomes cold.  Cold air is denser and heavier so it drops, pulling in fresh humid air from the attic or other airspace. As the air cools with the moisture condensing on the duct and it starts all over again.
Sealing the areas the ductwork goes up the walls at both ends is important and may be the root cause .
Your HVAC guy is a hack no one is perfect but they all say that. You just need to isolate any air flow and this is a common issue with multi story homes. The insulation is usually installed after the ductwork and sheetrock so they may not have sealed the chase but your hvac guy should have known this and told you to make sure the verticals shafts were sealed.