RouterBOARD mAP 2nD voltage

Dear, I put some mAP 2nD devices, to measure voltage at 54v, I have 5 in the network, but none accurately measures the voltage, at P4 it reaches 54v, but the same only measures 47v, 46v, what could you do to correct this fault?
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Suggest you send this directly to support@mikrotik.com to get manufacturer to address this.