Does the unevenness of the back during bowing invalidate the prayer or render it unacceptable?

If the lack of straightness in the back means a lack of ṭuma’nīnah (tranquility/stillness), then the prayer is invalid, based on the hadith of the one who prayed badly, which indicates the obligation of ṭuma’nīnah in all pillars of the prayer. However, if what is meant by the lack of straightness is that the back is not straight, but ṭuma’nīnah is present, then the prayer is valid, and that amount is considered sufficient for the rukūʿ (bowing), even if it lacks perfection.

The Sunnah in rukūʿ is to level the head with the lower back, to rest the hands on the knees while keeping them away from the sides, to spread the fingers over the knee and shin, and to flatten the back.

Therefore, a Muslim should strive to embody the Sunnah of the Prophet, peace and blessings be upon him, in their prayer.