I might say its the sudden change in heat, but I don't feel like that's the real reason. Swords can be kept with discus and I know discus take temperatures in that range and even higher at times, so I feel the heat isn't the biggest factor. More likely it could be that emersed grown leafs are melting off, you're plant is just melting due to change in environment, or you are missing either nutrients are do not have enough light and your sword is melting because of that. I bought this huge rose sword once, and I didn't really have proper lighting, so this huge rose sword lost all of its leafs and became nothing more than just a root ball. Now that I have higher lights but low nutrients, its still a root ball but it at least has a new crown or two of submersed growth that is small but steadily growing, and finally is putting out new roots.
What kind of lights are you using, that might be helpful for us to know.