At least I can now convert almost anything to stone with heat sim off
I looked at the code, and as much as it doesn't seem logical, it's intended. You see, when LAVA hardens back (or something, it's hard to understand this really old code), it will sometimes go into STNE instead. WIth real heat simulation default lava will also harden into stone, but in legacy I guess any kind can sometimes. Anyway, why would you ever want to play with heat sim off ... it's horribly buggy and nothing works as expected. I wouldn't want to change it since it's only there to keep old saves working in the same way.
very very odd ... I can't even understand how this even happens. I will look into it and post when I fix it, it seems to have just been broken recently to do this.