I used yesterday, in QGIS 3.2.3 the tool to convert multiline to a single line ("multipart to single part").
I chose the option of the command "multipart to single part" the geometry feature (line) and marked z dimensions. QGIS save right the temporary layer in format shape, a shapefile with z dimension included.
When I tried to eliminate z dimension with
ogr2ogr and inserted in QGIS, the program consider that the layer is a single line geometry. Who can help me?