when this happens to my javas they affect the host plant and plantlets. I've also done a lot of research and some say its a fert deficiency or excess of one or more macro or micro, low KG, low Mg, high temp, high light, co2, excel, salts,or even a virus or disease but nobody seems to be sure. in my case it first happened when I raised the temp and added salt to treat ich. However it still happens at 76-77F. One thing that's true is that the rhizome will sprout new growth after you trim off all the dead stuff. I definitely know its not high light since the ones that are really close to the top of the tank that receive the most light have not been affected (using 2xt5ho). I suspect that a melting leaf might "transmit" the melting to another leaf so cut off the dying leaf asap.
I really hope somebody comes up with a definte answer since there seems to be a lot of peeps affected by this.