Many of my phals have some kind of spot or another. To me, this looks a bit like an insect bite, or perhaps a slight edema, caused by the plant taking in more water than it can use (mine get them when they get rained on at nighttime and can't use all the water they are receiving. It's perfectly harmless.). It doesn't look like a disease to me, and the leaves look very, very healthy otherwise.
Keep an eye on it to see if it spreads, or if you can see any creepy crawlies (or jumpy wumpies). But, unless anyone else recognizes this as an illness, I don't think you have anything to worry about.
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