Help, weird "hard edged" appear on model Hard edges appear randomly on my model. The edges was not crease, and the normal was not flipped. It even remained after I performed smooth vertices. It only disappear/hide if I use subdivide modifier. I prefer not to use it to maintain a low poly count
Furthermore, it only appear on this particular part(the image shown is the only place it appear on the entire model.)
The following is the provided blend file

Any idea how to solve it ?
You've enabled Auto Smooth, it will sharpen all the angles that are over the chosen value, here 30°, so disable it or choose another value?