i found the problem the heat simulation is off if you turn on the heat simulation the titan will harden
Ok, it's because you have heat simulation turned off. I haven't really seen saves with HS turned off so I was unfamilier with how it works.
I think this save is just cursed. I can not replicate this effect outside of this save.
I must admit I have absolutely no idea.
of course it melts. Just like your brain
i put fresh ttan in the save and cut a hole in it and put fire
i experimented a bit at in seems like the molten titan can spread to other titan