Update: just found out that the area will change depending on the hour, so at 0:00, it will be directly on top of my character. From 8pm to 23pm, it is located in the west and from 1am to 5am, it's at the east. I think it's because how Bes designed the sky, it's a spherical with symmetry in 2 hemispheres, so even the sun doesn't come out at night, the setting fSunBaseSize for it still affect the night sky.
darkrain261's post in Strange interaction between fSunBaseSize + TAA with night sky was marked as the answer