Hi, swords, your case exactly tells me that the "rust" sign is due to some kind of fungus or virus. That one is infected, and with high humidity (I think your lowland tank has high humidity, no?) the "rust" sign will frequently appear. And cutting the top vine is one way to completely remove the fungus/virus. The other two N.amp are not infected, so they are not affected by the temperature and humidity.
I just wonder if anyone has gotten success in the use of fungicide or something like that to completely remove such "rust" sign?