im still new in using blender, i was following this video (https://www.youtube.com/watch?v=tI1uObIx4lM&ab_channel=3DJeffAllen) to make the model, but when i trying to add shoulder mesh to vertex group, the mesh become weird, the mesh has 2 mirror modifier (main & shoulder) and the one has problem is the shoulder mirror. I hope someone can help to solve this problem. Thank you very much.
https://drive.google.com/drive/folders/185745RvH2boslpRQP4kAoraoRG7oSOUK?usp=sharing