Hi liili,
1. What exposure time did you use?
2. What temperature did you shoot at?
3. Is this a stretched image, and what software did you use?
This is probably not much help, but here are examples of my Bias. Both are autostretched in PixInsight. Not the same thing, but they have their own odd features.
Left: single sub at -10C and 0.20sec.
Right: Master Bias integrated from 100 subs.
No one has said these bias have a problem.
Mark