Why is the word "سنّت" (sunnat) sometimes written with an open "tāʼ" (ت) and sometimes with a bound "tāʼ" (ة) in Mus'hafs, such as in "فقد مضت سنت الأولين" (the way of the ancients has passed)?

The fundamental principle concerning the recitation and the orthography of the Mushaf is tawqif (divinely ordained). The word "سنة" (Sunnah) falls under this principle. As for words like "رحمة" (rahmah - mercy), "نعمة" (ni'mah - blessing), and "كلمة" (kalimah - word), they were sometimes written with an open ta (ت) and sometimes with a closed ta (ة) due to the existence of two readings: one in the singular and the other in the plural. Thus, the orthography of the Mushaf accommodates both readings.