As with MicroStation, OpenCities Map allows transparency to be applied or denied to elements and objects at multiple entry points during the design process.
Users must determine the correct settings, applied at the necessary points to achieve desired output. Consider the following scenario: 

A group of elements have varying degrees of transparency assigned as per their element properties. 
When View Setting Transparency is engaged for the view from which we examine the elements, we expect and do see transparency applied. 
If then we duplicate display of the elements by creating a reference attachment of the same model to itself (offset to show side-by-side) we continue to see the expected transparency - the reference and the original appear identical in this regard. 

In addition to displaying the original transparency, the reference attachment affords the user an additional transparency component, applied to the overall attachment.
When set, this then further de-emphasizes the attachment's display accordingly, within the same view. 

However, when plotted in this state without further augmentation - the reference attachment transparency will not be applied. 

To cause the reference attachment's transparency setting to be applied to the print / plot output, select the desired attachment then from the reference dialog: Properties > Adjust Colors...

 

...and in the resulting Adjust Colors dialog, engage the Print Adjusted Colors setting:

 

Video example: