This was posted a long time ago by autodesk:
1. Close the parent drawing.
2. Open the child drawing, and set the INDEXCTL system variable to 2
or 3 to ensure that a spatial index is saved with the drawing.
3. Save and close the child drawing.
4. Open the parent drawing and see if the extents now calculates
properly. If the problem still occurs, you may need to recreate the
clipping boundary, as follows:
1. Use Xclip Delete to remove the clipping boundary in the parent
drawing.
2. Save, close, and reopen the parent drawing.
3. Use Xclip again to recreate the clipping boundary.
To avoid the problem in future drawings, be sure to set INDEXCTL to 2
or 3 in all drawings that you're likely to use as attached external
references.
Note that this workaround does not address the cases where a spline,
ellipse, or 3D solid falls on the Xclip boundary. For the purposes of
calculating the extents, AutoCAD treats such objects as if the whole
object were visible, rather than just the part of the object that lies
within the boundary.
For plotting, an easy workaround is to use Limits instead of Extents.
Set the drawing limits to the corners of the xclip border and then
plot limits.