Dezincification of brass is your answer. The alloy composition has changed some of the zinc has dissolved leaving only copper, its probably only the surface layer and a quick hand Polish of a case would show that. They should till be OK.
Worst case there will be voids in the crystal matrix and the hardness of the remaining metal and it's elasticity will be compromised but this is unlikely unless the citric acid concentration was very high and the treatment period really prolonged,.
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