Ductwork in Design Master HVAC can either be centered, or flat on the left or right side. See this blog post for more information on setting the horizontal offset for ducts.
When the offset changes in the middle of the ductwork, the ductwork ends up looking like this:
To prevent this, you need to insert a transparent duct at the break where the offset changes. The transparent duct is the one drawn in red in the following example.