I can't tell you the reason, but I have experimented a bit. I have found out I can write it by typing in, but the rhombs appear if I insert it (copied from your posting in greek letters) by pasting into the input box.
It might have to do something with the keyboard layout and mapping, or language setting in LibreCAD or your OS, but I am no guru on that.
I got some more success with greek_ol.