What is the ruling on the prayer of the Imam and the congregants if the Imam recites "iyyāka" in Surat Al-Fatihah without tashdīd?

The Fatihah must be recited with all its letters and its fourteen tashdidat (stress markers). Whoever omits a tashdid while being able to pronounce it, his prayer and the prayer of those behind him are invalidated. However, if he is unable to do so, his prayer is valid for himself, but there are two opinions regarding the validity of praying behind him.