I can imagine Judas hearing someone recounting a miracle and he'd be shaking his head and thinking "I was there, that's not what happened at all".
I don't think that does justice to the historical weight of the New Testament accounts. We have multiple eye witness accounts of the events surrounding Jesus' life, recorded within a few decades of the events themselves and emerging from an oral tradition that placed a premium on verifiable accuracy. The early Church writings we have (for example Eusebius) make it clear how concerned the first Christians were to ensure the historical reliability of their teaching, and how strongly they opposed the spreading of stories that were of doubtful veracity. In fact, the New Testament itself repeatedly tells its original readers to verify its accounts by asking other eye witnesses (e.g. Luke 1, 1 Corinthians 15).
e.g. Did Jesus exorcise demons?
As far as I am aware there are no contradictions on the Bible. People often point to the sorts of cases claiming a contradiction, when really it is two authors describing different aspects of a thing that are all simultaneously true.
More apropos would be the accounting of Judas's death: Matthew describes him as hanging himself, whereas Acts says he burst open in a field. However this isn't a contradiction either: it may simply be the case that he hung himself in a field, then over time grew bloated and burst open.
I honestly don't care if 1% or 5% of the text is real anyway.
I have heard a story that Judas didn't actually betray Jesus willingly. It had to happen this way and Judas just played his role. Makes the story more tragic and less black and white.
There's too many alternative explanations to say that either the miracles were false xor the Judas story was false - I don't see that they're necessarily connected.
The assumption here is that people will unconditionally believe and follow a person who performs miracles. But human beliefs and actions are more complex than that. People regularly refuse the evidence of their own senses, or that of knowledgeable authorities, if it contradicts their prior world view. They also regularly act contrary to better knowledge if they think it's in their interest.
So I think it's perfectly plausible that Judas experienced all of Jesus' miracles, but out of a mix of personal disappointment (Jesus not living up to Judas' expectations) and greed decided to betray him nevertheless.