The partial melting algorithm is being applied to transit tube crossings, heavy watt crossings, weight plates (and maybe more). These are all buildings that act as tiles.
Just build any of the buildings above (maybe more), and put something hot enough next to them to enable partial melting. You can watch the background element disappear (while the front building retains the full mass for display).
It is basically the same issue as insulation flaking (which was patched), but the patch seems to have missed buildings that act as tiles.