When is the word "rahmah" (mercy) written with a bound taa (ة) and when is it written with an open taa (ت)?

The obligatory is to follow the Mushaf's (Qur'an's written text) orthography in words like "rahmatun" (mercy) and similar ones. Readers might pause on them with a "hāʾ" (ة) or a "tāʾ" (ت). The word "rahmatun" (رحمة) appeared with an open "tāʾ" (ت) in seven places in the Holy Qur'an: Al-Baqarah (218), Al-A'raf (56), Hud (73), Maryam (2), Ar-Rum (50), and Az-Zukhruf (32). Al-Mutawalli compiled them in two verses from "Al-Lu'lu' Al-Manzūm."