Strange problem with metapost graphics

[Jean Magnan de Bornier <jm.bornier@free.fr>]

Hi all, 
Here is minimal example of some trouble I have:

...................................
\startMPinclusions
 numeric u; u=1cm;
\stopMPinclusions
\starttext
\startuseMPgraphic{dummy}
drawarrow (0,0)..(8u,0);
label.bot(\sometxt{temps}, (7u,-.5u));
\stopuseMPgraphic
\placefigure[fig:Uncontratdanslaflèchedutemps]{Un contrat dans la flèche du temps}{\useMPgraphic{dummy}}


\startuseMPgraphic{ummy}
  pair A[];
  A[0]:=(-1u, -1u);
  A[1]:=( 6u, -1u);
  A[2]:=( 6u,  4u);
  A[3]:=(-1u,  4u);
  draw A[0]--A[1]--A[2]--A[3]--cycle withpen pencircle scaled 1pt;
label.bot(\sometxt{Dupont}, A[0]);
label.top(\sometxt{Smith}, A[2]);
\stopuseMPgraphic
\placefigure[fig:Boîte d'Edgeworth]{Boîte d'Edgeworth}{\useMPgraphic{ummy}}
\stoptext
.................................

The trouble is that this compiles nicely with pdftex, a little less nicely
with xetex, and badly with luatex, tested at home and on context live.

- xetex: the label in the first graphic appears with bad size.

- luatex: first label in the second graphic appears with bad size and font,
and second label disappears; but if I comment out the label in the first
graphic, the second graphic is as expected.

Any idea?
-- 
Jean
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