Maybe I got answer  here.

  
      
      Deep Learning Course Forums – 24 Oct 19
  
  
    

Fluctuating performance of pre-trained model (frozen parameter) 11

Yeah, that looks normal. Note the scale, it’s only fluctuating in a pretty small range (only the last couple of epochs are in that range, so it’s preserved the final accuracy).   You’re missing the fact that f(x) may not be a deterministic function....