Answering my own question, it appears that GRASS does NOT polyline successfully even when there is only 1 attribute in the .dbf file.
When I tried a 1-attribute .dbf, the result was the same: using the mouse to query on a line segment brought up the Form displaying nothing for attribute values.
Looks like GRASS cannot create polylines, as Arc software does (which is to join line segments sharing a common attribute, such as street name).
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