The record
On September 27, 2017, a user describes dictation on their iPhone showing the right words as they talked, only for many of those words to change after they tapped Done. They say this had continued through multiple iOS updates and did not name a model or version. The original post does not list troubleshooting steps, and replies suggesting Siri checks and turning off predictive text did not produce a confirmed fix from the poster. The thread was later closed. This is one person's report and says nothing about how many users saw the same thing.
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A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.