Couple of things comes in my mind: perhaps problem is because layer mask is selected in the Layers panel. I can not remember exactly but I have experienced something similar in the past. Another thing to address is second sample point which is, it seems, measuring pure white spot which you can not re-color by dragging middle or bottom of the channels point to the right. It is probably better idea to use retouching tools to patch instead of trying to match surrounding colors using curves, even if you limit Curves to only that area. And by the way, 11 x 11 Average is perhaps too much, change from Sample Size drop down list to 3x3 Average, for example.
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