How to solve attenuation caused by filter coefficient quantization in Matlab?

Question

Before quantization of filter coefficient, it behaves well, but after quantization the amplitude drops to 0.52186, which is expected to be 1000(fig.1). But according to the amplitude response in filterDesigner(fig.2). The 200Hz wave shouldn't decrease so much after filtering. Could someone tell me why the abnormal attenuation happens? Why is it 0.52186?

Fig.1 effect of filter

Fig.2 amplitude response

Matlab code is as follows

N = 2000;
    f0=200;
    f1=800;
    fs=2000;
    n=1:N;
    y =1000*sin(2*pi*f0.*n/fs)+500*sin(2*pi*f1.*n/fs);
    Hd = filter_below500Hz;
    d = filter(Hd,y);
    figure(2)
    subplot(2,1,1)
    plot(y);
    title({['y，before filtering']});
    subplot(2,1,2)
    plot(d);
    title({['d，after filtering']});