ConTeXt : using \MPtext in different textbackgrounds

Question

I am trying to change my theorem environment, and I played a little with MetaPost (that I don't know well, being more of a TiKz user). I ended with this layout (which I really like) with the help of this manual, but the behavior doesn't fit : it uses the last \MPtext for all calls to the textbackground (see MWE below). When I place a big chunk of text and theorems in a bufer, the behavior is even worse. Is there a way to correctly handle this ?

Thank you in advance,

\startuseMPgraphic{mp:axiomframe}
  begingroup;
    numeric u; u := 1EmWidth;
    for i=1 upto nofmultipars :
      pair bl, br, ul, ur;
      bl := llcorner multipars[i]; br := lrcorner multipars[i];
      ul := ulcorner multipars[i]; ur := urcorner multipars[i];
      % Draw the surrounding box
      path p;
      p := ( bl -- br -- ur -- ul -- cycle ) enlarged (-0.5u,0.5u) ;
      fill p withcolor 0.95 white ;
      draw p withcolor 0.8 white ;
      draw (p cutbefore point 0 of p cutafter point 2 of p) withpen pencircle scaled 1.2bp withcolor 0.8 white ;
      p := ( ul -- (ul shifted (0,-2u)) -- (ur shifted (0,-2u)) -- ur -- cycle) enlarged (-0.5u,0.5u);
      picture lab;
      lab = thelabel.lrt(\MPstring{TitreTh},ul) shifted (2u,u);
      p := (llcorner lab -- lrcorner lab -- urcorner lab -- ulcorner lab -- cycle) enlarged (0.5u,0.5u);
      draw p withpen pencircle scaled 1.2bp withcolor 0 white ;
      fill p withcolor 0.8 white;
      draw lab; 
    endfor ;
  setbounds currentpicture to OverlayBox ;
  endgroup;
\stopuseMPgraphic
\definetextbackground
  [theoremFrame]
  [mp=mp:axiomframe,
    location=always,
    leftoffset=1em,rightoffset=1em,
    topoffset=0ex,bottomoffset=0ex,
    frameoffset=5\bodyfontsize,
    before={\testpage[3]\blank},
    after={\blank[medium]}]
\def\starttheorem[#1]{
  \setMPtext {TitreTh} {\bf #1}
  \starttextbackground[theoremFrame]
  \vskip 1em
}
\def\stoptheorem{\flushsidefloats\stoptextbackground\\blank[big]}
\starttext
\starttheorem[Knuth]
  \input knuth
\stoptheorem
\vskip 2em
\starttheorem[A formula]
  \startformula
    a + c = x
  \stopformula
\stoptheorem
\stoptext