March 2017 - last edited March 2017
I'm pretty sure this is by design... the space explorer armor is a set, so it includes legs and arms when equipped. You can change your helmet, but the rest is included in the item.
If I remember, the chest sort of functions as a full body piece, except the helmet. As you progress, you'll see some armor shows an icon of a full body suit vs just a chest piece. Armor like that doesn't tend to have arm or leg slots available, I think.