If the Witr qunut is omitted by mistake in the third rakah and the prayer otherwise continues, sujud al-sahw is due because qunut is wajib in Hanafi law.
Omitting a particular recommended wording should not automatically be equated with omitting qunut altogether; the required act and the selected supplication wording have distinct rules.
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Prostrations of Forgetfulness: Causes, Method, Missed Sittings and Doubts
If the Witr qunut is omitted by mistake in the third rakah and the prayer otherwise continues, sujud al-sahw is due because qunut is wajib in Hanafi law.
Omitting a particular recommended wording should not automatically be equated with omitting qunut altogether; the required act and the selected supplication wording have distinct rules.