Is this a phal leaf? Could be bacteria. You say its not mushy (yet). Has it been growing / spreading. Appears to be on the move with the yellow haze. Leaf looks pretty light green, due to getting lots of light. Dont know if that has anything to do here, but an observation.
You could cut that part of the leaf off and apply an antimicrobial to be on the safe side. If it doesn't exist already, we need a gallery of photos of common pests and ailments.
I agree it looks like sunburn. I would cut the leaf down to good tissue, getting rid of all the trouble area. Then you can simply put ground cinnamon from your spice rack on the cut edge, and that should take care of it. Cinnamon is an excellent anti fungal, and very cheap to obtain!
I have lots of Phals amd that looks ike sunburn to me too. I accidentally got one of mine in a window with too much light and got a spot like this one. The leaf was so damaged that even after I moved it the leaf continued to deteriorate until I finally removed it. LLW
Better a late response than no response ... the spot is probably sunburn but you can also get blemishes like this when water remains on a plant after sun goes down (temps cool off) or if cold water drips on a plant leaf.