My bard successfully convinced Rugan that she was sent to retrieve the shipment, but the lock symbol on the chest remained red, and Rugan would turn hostile if I tried to open the chest.
I reloaded my previous save and took a long rest before trying to open it again, hoping that Rugan and Olly would have left, leaving the chest behind. They were indeed gone - but so was the chest.
(Or maybe it's not a bug, and Rugan and Olly just realised that it's not a good idea to entrust their shipment to a random stranger

)