Currently Offset allows only one single segment of a polyline or polycurve to be offset at a time. If multiple connected subcurves are preselected, the selection is ignored*.
My wish is that this should be treated like ExtractSubCrv with multiple continuous segments and then offsetting the extracted curve (and deleting the original).
*A slight bug in the current versions (V7/WIP) is that if multiple subcurves are preselected, then Offset is run, the selection is ignored, the command line askes you to select a curve to offset… but the pre-selected segments remain highlighted as if they were active.