The record
A September 26, 2019 MacRumors forum post describes intermittent Siri failures on a 44 mm Apple Watch Series 4 paired with an iPhone SE running iOS 13. The author reports that weather, calculation, and web requests sometimes stall or produce incomplete spoken replies. They describe trying resets and different connections. The user says Siri had worked more reliably before the update; this is their reported comparison, not a controlled regression test.
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A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.