They are perma-gone. They only appear again if I build another component, and then that's only while the 3-D scaffold graphic shows and then it all disappears again.

I put the debug.err up a little later on, when I can get into the game again and play for a few turns to reproduce this.

If you are on the cloth map, and open the 'Build Structures' window, all icons show.  Until you close that window, that is, and only if you open it while on the cloth map.  If you are zoomed in to the 3d map, open the Build window, and zoom out to the cloth map, they do not show.