The record
On March 26, 2018, a user writes that HomeKit voice commands to their HomePod for Philips Hue lights usually worked, but every few days Siri would answer that some devices were not responding. They say the same lights kept working from the Home app and from Siri on their iPhone, pointing to the HomePod. Power cycling the speaker reliably cleared it, but the problem returned with no clear trigger, and no lasting fix was reported before the thread closed. This is an individual report and does not indicate a failure rate.
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A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.