After importing the model, delete the face of the mesh type component

Hi.
In the imported model, the component is a mesh type. After obtaining the index, what is used to determine which surface the data in the index belongs to.
ps: Tried to create a mesh in three, its index is fixed, 0-6 is always top
https://codepen.io/miao-liu/pen/PoaKyXG

Tested with the imported model, it seems that 0-6 is not always top



solved.
caused by uv