It can still be sunburn,and its spreading,I would cut the leaf say an inch below the spot,spray it with some hydrogen peroxide and then dust it with some powdered cinnamon to seal the area and prevent it spreading further. By the way does your phal get direct sunshine? Maybe what happen is you watered and water was left on the leaf and then the sun hit it in that spot causing the burning,which caused the leaf tissues to collapse so because that happen it cause a sort of infection which spreads inside the leaf eating the healthy tissue. But even if its not sunburn the hydrogen peroxide and cinnamon should stop it from spreading.was this the only leaf affected, or plant for that matter? Where do you keep it? If you provide a bit more information I am sure the experts here will be better able to help you,I hope this helped.Velleta

It's the only leaf on that planr that has it. The other phal is right next to it and seems happy as can be. They're both on top of a file cabinet that sits between two windows that get filtered light in the morning and some in the afternoon. I soak their pots in water and dry the crwons while they drain if any water gets down there. I didn't know if it was something more because of the pattern that's forming in the spot.

Spot looks like sunburn. But, why is it the only leaf? That suggests something else going on.

Jane