Question Backporting vertex skinning shader to built in

I'm doing this more as a temporary thing to allow me to move forward with integrating the new DOTS animation stuff as it's going to be a couple of months at least before we can move to HDRP.

The node I'm porting is here, it does vertex skinning for skinned meshes.
https://github.com/Unity-Technologies/ScriptableRenderPipeline/blob/master/com.unity.shadergraph/Editor/Data/Nodes/Vertex Skinning/LinearBlendSkinningNode.cs

This is a surface shader that implements the core logic.
https://gist.github.com/gamemachine/2fe6e1f919f6fa6cf5b20eb942ddc0da

And it works to a point. But it exhibits what looks like timing issues, like HDRP is interpolating the results over time somehow using a mechanism I'm just not aware of.

What happens visually is with a character comprised of multiple skinned meshes, they all get out of sync in relation to each other. Individual meshes the skinning is fine. The problem worsens the more meshes that are being rendered, and how off individual meshes are and how varies over time also. Which makes me fairly certain it's time related. What I don't know is if it's actually a shader thing, or maybe meshes are batched differently with HDRP and that somehow causes the issue.

A single character for instance I couldn't see any issues. It gradually gets worse with more.

Rendering is done via Graphics.DrawMeshInstanced, it's just the unmodified Dots Animation code.

 

Are the BoneIndexOffset values the same for all of the meshes? If not they'd each be using their own skinning matrices.

 

You know that's probably it, because I also just realized it's rendering via the hybrid renderer, which does not support instanced props for built in.