But in the days when the book of Acts was written, had they already witnessed a century of false predictions?
Yes. Well close to it. When Acts was written there was already a huge diversity of Christianities (a half dozen or so sects are cleverly referenced) and many of them evolved specifically due to the failure of the promised end. In fact, Acts was written in response to this. The focus had shifted from imminent judgement of the nations to forgiveness of sins and resurrection. The only brief mention of Jesus' return I can recall is the cryptically worded Acts 1:11 that lent to a multitude of interpretations.