Damn, hiatus 😭
Personally I don't agree that Jake's change was sudden.
He got to experience first hand how terrifying it feels to be stalked and "protected" without wishing for it or actually hoping it doesn't happen at all. That was all during the hunting arc.
Then everyone went to save Yuri and he got abandoned for a while. He had plenty of time for self reflection and repentance. Melissa told him upfront how terrible his behaviour was, his beloved weapon got destroyed, he was left alone, confused and defenseless. And all due to his own choices.
So...the maturity makes sense.
However, him being nice to Melissa makes less sense, and I'm inclined to believe that there's another outside influence behind that.🤔
Oh well, we won't know more until season 2, it's all speculation 😅