Well, if its a problem with the software, then it must happen between 16.2 and 17, or the back-translation stripped out the bad bits.
Did you apply the patches to bring your version 17 up to 17.2 ?
I just selected the entire set of lines, clicked on the double-mushroom icon, and then the finish flag, and it was joined. I was then able to simple extrude the shape and it formed a 3D object (not a ribbon).
I tried it in 2D/3D mode and 3D mode, and I tried the join tool in 2D mode and 3d mode, and every which way it seemed to work.
Just for fun, you might see if you can take the version you made for me and load it into 17 and then apply the same process. Its possible that there were some anomalous entities in your original that got cleaned out in the conversion process.