Modular Character Issues with SetMasterPoseComponent

I am using Epic free asset Stylized Character Kit to test my modular system.
At first, i used the Master Pose Componentnt method.
The Chest part assign to Character Mesh and set anim
Other part setupAttachment to main mesh, and setmasterposecomponent.
But it looks bad with charactert shadow

321585-16049767461.png

And Blueprint and C++ Impl, they all have this problem.

then i try Skeletal Mesh Merge method.

and the result looks good.

321578-微信截图-20201110105803.png

So i want to know, is anything i miss with Master Pose Component?