When you run the HIDE command, ductwork that is underneath a circle is drawn hidden. This most often occurs when the “Show Background Below Ductwork” option is set to “Yes.”
Unfortunately, this is a feature of the AutoCAD HIDE command. AutoCAD sees circles as solids, and anything below them is hidden. If you would like to see this behavior changed, you will need to contact Autodesk. It is not something that we can fix in our software.
Our recommended workaround for this problem is to draw the circle using two arcs. It is easiest to specify the ARC by inputing the center point, then two points opposite each other to create half the circle. Then use the mirror command to create the other half of the circle.
If the circles are on an XREF provided by someone else (typically the architect), it is a little more difficult to workaround this problem. You will need to insert the XREF twice in your drawing–once underneath your ductwork and once somewhere else. You then need two viewports in paper space. The first will show the ductwork and XREF. The second will show just the XREF. These need to be directly on top of each other so that the two copies of the XREF exactly line up. The XREF below the ductwork in the first viewport will be hidden, but it will appear in the second.