Im trying to recompute the normal with the computenormal vex function in order to get the same result as using a Normal sop node. But no matter how I write the code, I always get a wrong normal showing on the back faces of the geo.

I also can't make that function work. I always get a normal of {0, 0, 0} on all points.

What's the reason for not using the normal sop?

The docs say it's for use in shading context and that it's obsolete for SOP context.

in Point Wrangle:

- if you don't have N attribute on polygonal geo yet, then simply writing
will create it based on topology

- if you already have N attribute, then you can check Bindings/Update Normals If Displaced
but then you need to modify v@P for it to trigger such update, so if your Wrangle doesn't modify v@P, you can do at least this to trigger the update: