How to fix lag of rotation of one bone from another in Blender rig (video)?

Question

3D

Hello. I'm trying to master the rig and animation, I looked and tried to repeat a simple (as it seemed to me) tutorial on this topic https://www.youtube.com/watch?v=jX7eLBEPCEc&t=353s... and something goes wrong, although I repeat everything seems to be exactly right. For some reason, the mesh attached to the humerus lags behind during rotation, I recorded a video https://www.youtube.com/watch?v=Xd3ufCXmasQ&ab_cha...
I tried to distribute the weights and how in the splint through the parent bone - the result is the same same. I would be grateful for any direction in which you can search, it seems to me that I have already moved all the sliders and everywhere))