What is the ruling on praying behind an imam who does not observe one of the pillars well such as not putting nose on the ground? Is the prayer of the followers valid?Pls do not refer me to other FATWA

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The Prayer is not invalidated by not placing the nose on the ground in prostration. Placing it on the ground is not one of the pillars of prayer or one of its obligations, but rather, a desirable Sunnah according to the view of most scholars.

Ibn Qudamah said in Al-Mughni:

“In regard to the nose, there are two narrations: one stipulates that a person has to prostrate on it, and the second narration stipulates that it is not an obligation to prostrate on it. This is the view of ‘Ataa’, Tawoos, ‘Ikrimah, Al-Hassan, Ibn Sireen, Ash-Shafi’i, Abu Thawr, and the two companions of Abu Haneefah, because the Prophet said: “I was ordered to prostrate on the seven limbs (body parts).”; but he  did not mention the nose.

It was also narrated from Jabir that he said: “I saw the Prophet prostrating on the uppermost of his forehead on parts of (his) hair.” [End of quote]

Therefore, there is nothing wrong in imitating the Imam you referred to in the question, although it is more appropriate for him to prostrate on his nose along with his forehead.

Allah knows best.