Draw to render target comes out always black


I’m trying to replicate what Ryan Brucks did with dynamic damage to characters: Character Damage using Render Targets in UE4 - YouTube

I have the first part (encoding local positions to a render target), but I’m not able to draw to a render target if I use those local positions as input for the painter material.

This is the material I use to paint the the render target on hit position (local):

When I set the viewport to render the material on the character it correctly paints the area around the hit:

But when I use this material to draw to render target (or when I set the view port to render the material on a plane too) it comes back pitch black:

Does anyone know what I’m doing wrong?