Hie ,
This looks pretty much the very same issue as the issue in this thread: https://.unrealengine.com/showthread.php?78885-Animation-Blend-Per-Bool-Issue-Weird-Pop-or-Jump-or-something
4.8.3:
*) Create New Project -> Third person
*) Switch back to library and do “Animation Starter Pack” -> Add to project (looks like Kanc is using some ninja anim at this point instead)
*) Setup retargetting for UE4_Mannequin_Skeleton found in Content\ThirdPersonBP\Charater\Mesh to Humanoid Rig and switch pelvis bone to Animation scaled
*) Setup retargetting for UE4_Mannequin_Skeleton found in Content\AnimStarterPack\UE4_Mannequin\Mesh to Humanoid Rig
*) Retarget any of the animations found in Content\AnimStarterPack for example Idle_Pistol or Idle_Rifle_Ironsight
*) Create a 1D blendspace for your skeleton found in Content\ThirdPersonBP\Charater\Mesh from ThirdPersonIdle to Idle_Pistol_Copy and make Bones visible.