Model with weapon retargeting issue

Hello, i have a character that holding a knife but weapon has bones attached to the “Spine” bone. When i try to retarget it, weapon flying because i don’t know how to fix or retarget it. Here you can see how they modeled the character;

@Galasdaer were you able to resolve this issue? I found this video that might be useful to you: https://www.youtube.com/watch?v=pa9AjL36wdY&ab_channel=OwennN