What is the ruling on pronouncing "Maliki Yawm al-Din" in Surah Al-Fatihah with a sukoon on the letter "Kaf" instead of a kasra, and does this affect the meaning during prayer?

Voicelessness (sukun) of the letter 'kaf' in the word "Maliki Yawm al-Din" is not a melodic error (lahn) that alters the meaning. However, it is disliked (makruh) and intentionally doing so is forbidden (haram). Nevertheless, the prayer of one who intentionally does so is not invalidated.