If ablution is broken between the obligatory and sunnah prayers, is it necessary to repeat the previously performed…

The prayer begins with the opening takbir (iftitâh tekbiri) and ends with the salutation (salâm). It is necessary not to neglect any of the conditions required for the validity of the prayer between these two points. For example, being in a state of ablution is one of the conditions of prayer, and this state must be maintained until the end of the prayer. However, once a prayer is completed by giving the salâm, any situation that occurs afterward does not affect the prayer that has already been performed.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu