richedit with EM_SETCHARFORMAT SCF_SELECTION work until 256 ?

hello, im learning unicode richedit control like this

Code:

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        CString data(_T("Data"));
       
        s_richedit.Append(data);  // s_richedit is CString control of rich edit

        CHARFORMAT2 cf;
        ::memset(&cf, 0, sizeof(cf));
        cf.cbSize = sizeof(cf);
        cf.crTextColor = RGB(0,0,255);
        cf.dwMask = CFM_COLOR ;

        ctl_richedit.SetSel(ctl_richedit.GetTextLength(), ctl_richedit.GetTextLength() + data.GetLength()); //ctl_richedit is CRichEditCtrl of richedit

        ::SendMessage(ctl_richedit.GetSafeHwnd(), EM_SETCHARFORMAT, SCF_SELECTION, (LPARAM) &cf);
        ctl_richedit.ReplaceSel(data);

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actually that works, but when the size of richedit get around 256, the color back to BLACK
what's going on here? i had tried to replace

Code:

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        ctl_richedit.SetSel(ctl_richedit.GetTextLength(), ctl_richedit.GetTextLength() + data.GetLength()); //ctl_richedit is CRichEditCtrl of richedit
with
        ctl_richedit.SetSel(ctl_richedit.GetTextLength(), 500); //ctl_richedit is CRichEditCtrl of richedit

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nothing change :D

thx for any opinion and help ^^