Possibly an insect probed into that section (eg. sucking insect got into there for a little while only and is now gone) ----- just a maybe only. Hard to say what actually happened --- but as long as that part doesn't spread, and if it dries up and remains hard - then it'll be ok.
Maybe just keep that leaf dry (or allow it to stay dry) ----- and keep monitoring each day.
The circled portions (in the attached pic) appear to be little chunks taken out of the leaf.
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