This issue might be linked to this Vector CrossProduct bug?
Still I am not sure so feel free to delete or move this.
Please see the image below
Basically, I expect that a cross product of two opposed normalized vector v1 = (0,0,-1) and v2 = (0,0,1) will give us v3 = (0,0,0).
The cross product in gh seems to be a bit off when you activate the unitized option.
All the best and keep up the good work,