I’ve made a simple example.
I have lowpoly head with UV1 and hipoly head with UV2.
When i try to transfer texture from hipoy to lowpoly, i received texture with total mess.
Look to my tree
- models are aligned.

- destination has uv

- source has uv

- destination is corrected

- source is corrected

- transfer texture is broken

- my destination has inner parts (tongue, eyelashes,eyes e t.c. ). When i remove them, i receive UV projection, similar to desired, but 70% of surface leave untextured.

How to fix it?