Contradiction with Luke 19:10
Isaiah 26:15 speaks of enlarging the nation and increasing its joy, whereas Luke 19:10 focuses on seeking and saving the lost, implying a different purpose.
Luke 19:10: For the Son of man is come to seek and to save that which was lost.
Contradiction with John 3:30
Isaiah 26:15 talks about the expansion of a people, while John 3:30 emphasizes the decrease of oneself for the increase of another.
John 3:30: He must increase, but I [must] decrease.