The record
In a post dated January 13, 2021, a user with an iPad (5th generation) Wi-Fi on software version 14.3 says Siri answered normally when invoked with the Home button but ignored the spoken wake phrase. They add that the microphone seemed fine for other uses, though a FaceTime caller had mentioned their audio cutting in and out. They report going through the suggested checks and restarting the iPad several times without a fix. This is an individual report and may reflect conditions specific to their device.
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A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.