How can i customise the breakable box

Question

In my example below, I have a problem with my breakable box, he does not want to ride.

\documentclass[12pt,a4paper,oneside,openright]{book}
    \usepackage[utf8]{inputenc}
    \usepackage[frenchb]{babel}
    \usepackage[most]{tcolorbox}
    \usepackage{multicol}
    \usepackage{lipsum}
    \usepackage[left=4cm,right=3cm,top=3cm,bottom=3cm]{geometry}

    \usepackage{etoolbox}
    \definecolor{tcbcolback}{RGB}{255,128,0}

     \usepackage{varwidth}
    \newtcolorbox{mybox}[2][]{enhanced,
        before skip=0mm,after skip=0mm,
        colback=black!5,colframe=black!50,boxrule=0.2mm,
        height fixed for=first and middle,
        attach boxed title to top left={xshift=1cm,yshift*=1mm-\tcboxedtitleheight},
        varwidth boxed title*=-3cm,boxrule=1pt,breakable,pad at break*=0mm,colframe=green!75!black,drop lifted shadow,
        boxed title style={frame code={
                \path[fill=tcbcolback!60!black]
                ([yshift=-1mm,xshift=-1mm]frame.north west)
                arc[start angle=0,end angle=180,radius=1mm]
                ([yshift=-1mm,xshift=1mm]frame.north east)
                arc[start angle=180,end angle=0,radius=1mm];
                \path[left color=tcbcolback!90!black,right color=tcbcolback!90!black,
                middle color=tcbcolback!99!black]
                ([xshift=-2mm]frame.north west) -- ([xshift=2mm]frame.north east)
                [rounded corners=1mm]-- ([xshift=1mm,yshift=-1mm]frame.north east)
                -- (frame.south east) -- (frame.south west)
                -- ([xshift=-1mm,yshift=-1mm]frame.north west)
                [sharp corners]-- cycle;
            },interior engine=empty,
        },
        fonttitle=\bfseries,
        title={#2},#1}

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \NewTColorBox[auto counter,number within=chapter]{exercise}{m+O{}}{%
        enhanced jigsaw,enforce breakable,pad at break=0mm,break at=-20pt,
        colframe=green!20!black,
        colback=yellow!10!white,
        coltitle=green!40!black,
        fonttitle=\bfseries,
        underlay={\begin{tcbclipinterior}
                \shade[inner color=green!80!yellow,outer color=yellow!10!white]
                (interior.north west) circle (2cm);
                \draw[help lines,step=5mm,yellow!80!black,shift={(interior.north west)}]
                (interior.south west) grid (interior.north east);
        \end{tcbclipinterior}},
        title={Exercise~ \thetcbcounter:},
        label={exercise:#1},
        attach title to upper=\quad,
        after upper={\par\hfill\textcolor{green!40!black}%
            {\itshape Solution on page~\pageref{solution:#1}}},
        lowerbox=ignored,
        savelowerto=solutions/exercise-\thetcbcounter.tex,
        record={\string\solution{#1}{solutions/exercise-\thetcbcounter.tex}},
        #2
    }

    \NewTotalTColorBox{\solution}{mm}{%
        enhanced,
        colframe=red!20!black,
        colback=yellow!10!white,
        coltitle=red!40!black,
        fonttitle=\bfseries,
        underlay={\begin{tcbclipinterior}
                \shade[inner color=red!50!yellow,outer color=yellow!10!white]
                (interior.north west) circle (2cm);
                \draw[help lines,step=5mm,yellow!80!black,shift={(interior.north west)}]
                (interior.south west) grid (interior.north east);
        \end{tcbclipinterior}},
        title={Solution of Exercise~\ref{exercise:#1} on page~\pageref{exercise:#1}:},
        phantomlabel={solution:#1},
        attach title to upper=\par,
    }{\input{#2}}

    \tcbset{no solution/.style={no recording,after upper=}}
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    \begin{document}
    \begin{mybox}[]{Exercises }
    \begin{exercise}{Ex1}[coltitle=cyan!80!black]
            \lipsum[1-2]
    \end{exercise}  
    \begin{exercise}{Ex2}[coltitle=cyan!80!black]
        \lipsum[1-6]
    \end{exercise}
    \end{mybox}
    \end{document}