Deselect export constructor shown in picture. You will get a new menu.
Make sure "Store gloss/roughness in Metal alpha is deselected.
This will export all your maps correctly.
FYI. Currently the new Principled Shader in Blender Cycles appears not to read the roughness map as we see it in 3DC. I had to adjust my roughness map externally to match 3DC's viewport.
Yes, something appears not quite right with the export constructor for Blender Cycles.