When making a GET_FEATURES request on an ArcMap Service and requesting the envelope (envelope = "true") the envelope is not returned for the target layer.
ArcMap Server is looking for the #SHAPE# field in the list of subfields of target layer in order to return the envelope.
<BUFFER distance="0.1" bufferunits="miles">
<TARGETLAYER id="1" />
<SPATIALQUERY subfields="#SHAPE# NAME" />