AutoCAD 2013 paper space viewport oddity

This is an odd issue.  I have a drawing with some 3d objects that I display with multiple viewports, in paper space, showing different views of the objects.  The viewports themselves are all on one layer that is frozen.  Each viewport with a 3d object is set with Visual style: Hidden & Shade plot: 3D Hidden, to hide all lines that should not display for that view and creating a nice clean image in each viewport.  Again, viewport layer is frozen so no viewport borders are displayed.  The problem I'm having is that I get random viewport border lines that show up on my plot.  It can be paper or pdf. and could be top in one viewport and side in another, etc.  I have never had this happen before but it seems to be related to hidding 3d line because I have never had a problem when I didn't.  Has anyone else had this issue?  Any Idea why or how to fix?

Thanks

MH

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