Looks like exactly what happened to my Bersa 45 UC. Started to chip at the rear of the slide under the hammer, like yours. As well as a bit under the slide, beneath the firing pin housing and a bit under the front end of the slide.
Not a huge deal (aside from cosmetics) and can pretty much be expected. Told the buyer when I sold it. Was of no concern to him at all and took no loss on price from it.
It should stop chipping as soon as it wears a bit.
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