Atheist Advocate (AA): Doesn't Christ admit that his witness of himself is true, and then later that it's untrue?

Christ's witness of himself is true [John 8:14-18]
"14 Jesus answered, “Even if I testify on my own behalf, my testimony is valid, for I know where I came from and where I am going. But you have no idea where I come from or where I am going. 15 You judge by human standards; I pass judgment on no one. 16 But if I do judge, my decisions are true, because I am not alone. I stand with the Father, who sent me. 17 In your own Law it is written that the testimony of two witnesses is true. 18 I am one who testifies for myself; my other witness is the Father, who sent me.”

Christ's witness of himself is not true [John 5:31]
"31 “If I testify about myself, my testimony is not true."

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