It is necessary to put geometry on the FG/BG layer, and to note the plug-in that reads information on the BG polygon.

It is a phenomenon that occurs in LW8.?-LW9 :

The polygon is put on the FG layer.
The polygon is put on the BG layer.
When the polygon of the FG layer has been selected, the BG polygon cannot be normally read.
When the point of the FG layer has been selected, the BG point cannot be normally read.
It is a thing with insufficient read number.
However, BG's spline curve can be read.

The FG polygon is selected, and the plug-in that refers to the BG polygon face doesn't become something useful.

Test environment :
LW8.5(win32)build 700 Japanese version
LW9(win32) non-public beta build 945 Japanese version

The BG polygon is scanned with MeshDataEdit.
err = edit->pointScan( edit->state, point_scanfunc, userdata, OPLYR_BG )
err = edit->polyScan( edit->state, polygon_scanfunc, userdata, OPLYR_BG )
In the command class, it is not possible to read normally excluding OPSEL_GLOBAL.