Question Light bias not affecting instanced meshes

Hello again,
I am having trouble another time with instanced meshes. This time, it appear that the light bias is ignored by those meshes, leading to undesired results.
Left are instanced meshes, right the non instanced.

They both are constructed by billboarding quads, and they both have the same self-shadowing problem. The difference is that I can get rid in the non-instanced model by increasing a little the shadow bias, but the instanced ones ignore it completely.
I am using a surface shader with a shadow pass. What is missing in my shader?