Voltage measured by a voltmeter is not the same as voltage across the resistive load of the miner.

Wall warts frequently fail in a way where they have correct voltage across no load like a multimeter but are unable to supply enough current at that voltage causing the voltage to drop under load.

You can measure on the pads of the barrel connection on the board while it is running to see if the voltage is lower than you measured on the power supply with no load.

Too complicated? You can just try another known good power supply with the same specs.

TLDR, or didn't understand, it might still be the power supply even if voltage looks good on a multimeter.