My character picks backpack and in blender it gots glued to his back and it moves exactly with spine bone, but in unity it doesn’t glue to the back, I would say it follows it fast and smooth but you can see that the backpack is floating and it’s not sticked to the back. In blender I use “child of” constraint to glue backpack to the character and it works perfectly in blender. Scale is 1/1/1 on rig and also on model.
In Unity, what does the setup look like? Actually find the spine bone and make the backpack a real child Transform to the spine bone. There should be no lag there.
I am surprised the Blender->FBX->Unity export chain does anything at all with Blender Object Constraints.