ولو كان ذلك الجزء ناقصاً كما في صلاة العصر فإن آخر الوقت وقت احمرار الشمس
Asr may enter a deficient, disapproved portion of its own prayer period near sunset.
Unlike Fajr at sunrise, the problematic state here can occur before the prescribed time has fully ended. The author uses that difference to explain different legal consequences for a prayer performed at the end of its window.
For Fajr, sunrise ends the period; for Asr, a disapproved quality can arise while the period is still running. The legal character of the time, not simply proximity to its endpoint, matters.