What is the ruling on lowering the head and bowing while standing in prayer, and does that affect its validity?

There is no objection to the worshiper lowering their head during the standing posture (qiyam) in prayer, as long as their back remains straight. It is not a condition to keep the neck upright; rather, some jurists have even recommended lowering the head as it helps one to look at the place of prostration. The noble Prophetic tradition mentioned in this context pertains to straightening the back in bowing (ruku') and prostration (sujud), not during the standing posture.