HDRP Lighting Normals Issue

I am attempting to bring a character into the HDRP and have ran into some weird shadowing issues with my model.
The character is getting a weird "chinstrap" effect when being lit, as seen below (standard view/single shadow only view)


The head on the left is a version of the model with double sided faces, the right head is the original with only faces pointing outwards.

From the way the shadow is being cast it seems like a normals issue, but when debugging the vertex normals in Unity they look fine:


We have tried a number of changes to the UV's and normals on the model, but still get the same chinstrap issue.

Does anyone know what could be causing an effect like this, or have any suggestions we could try? We are stumped as to what to try next