TCA9535: TCA9535PWR-- Lead Color Change Observed After Reflow Soldering

Part Number: TCA9535

I have provided feedback on the TCA9535PWR, A problem has occurred in PB: during the reflow soldering process, the component leads change color(Turning black and dark) from their original plating color...

Please refer to the picture below!

May I ask why? Is it normal?

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