Thanks for your reply Vecherka, I get your point, originally I had the same thought.
I tired your option Merge by Distance, looks like there is no “overlapped” vertices. I didn’t use any modifier. Just FYI I’m very new to blender. What I did test was just create basic mesh like uv sphere, and then export.
I also found that among those starter content meshes, only surface with “curve” edge does not have crank issue. faces like cube, pyramid, two ends of a cylinder/pipe also cracks when disable “crack free displacement”
