I just gave this a quick try in AutoCAD Mechanical 2010 and 2011 and this is working for me. Select the viewport > Right-click > Properties > Set Shade Plot to Conceptual or Realistic. After I save/close/reopen and/or plot the setting still persists.
Which version are you seeing this on, and what dwg format are you saving to? Also how are you creating these viewports? Are these vanilla AutoCAD VPORTS, or Mechanical AMVPORTS or ? Can you attach a dwg that has this problem?
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