What is the truth about Jesus in India during the "lost years" of scripture between His youth and His ministry?
Paramhansa Yogananda definitely said that Jesus traveled to the Far East to study with the great masters of India and Tibet.
This is a passage written by Yogananda, published in East-West Magazine, March-April, 1930 edition:
"Jesus was an Oriental, by birth, blood, and training. The Wise Men of the East, or East India, came to confer about him when he was born, knowing him to be one of the greatest message-bearers of Truth.
"...it has been definitely proven that Jesus was connected with the High Initiates and the Masters of India.
"In The Unknown Life of Christ, by Nicholas Notovitch, the Russian author tells how he went to Tibet, hoping to study Tibetan literature, hand-written on papyrus scroll. Because of the secretiveness of the Lamas, it was impossible to find a trace of what he wanted; but a strange miracle happened.
"Just when he was returning fruitlessly to India, he fell from a cliff and broke his leg, and was taken back to the monastery to receive the necessary care! The Tibetans are very hospitable to their invited or stricken guests. While the injured man was recovering, the head Lama asked him what he wanted.
"He said, 'Read to me the papyrus scrolls!' From these sacred scrolls he secured conclusive evidence that Jesus Christ’s name was Isa, meaning 'Lord,' which afterwards was pronounced as Jesus. He [Jesus] conferred with the Masters on yoga and great problems of human upliftment, living with them at the monastery...."
Your question asks: What is the truth about this matter? Personally I believe what Yogananda says about these "missing years" in the life of Jesus is true; but of course, that is up to you to decide yourself.
Hoping this helps!