Why the metapost not updated the value of text variables that are generated inside luacode environment?

In the code below, If I have \startitemize[columns,two] then metapost draw:

1. \Bolha[size=6cm]{text1}{text2}{text3}, 

2. and four  \Bolha[size=4cm]{text1}{text2}{text3}, but text1, text2 and text3 is not updated in every call;

If I omit the itemize environment or with \startitemize  metapost only draw:

1. the first call of Bolha that is: \Bolha[size=6cm]{text1}{text2}{text3}

2. and four call of \Bolha[size=6cm]{text1}{text2}{text3} %% all new information is omited

Jorge

%%%%% code

\def\Bolha[#1]#2#3#4%

{\setupMPvariables[bolha][#1]%

\setMPtext{labelcima}{#3}%

\setMPtext{labelesquerda}{#2}%

\setMPtext{labeldireita}{#4}%

\useMPgraphic{bolha}}

\startuniqueMPgraphic{bolha}

path p, q, cima, esquerda, direita, vertical; pair A, B, C;

p:=fullcircle scaled \MPvar{size};

A:= (point 4 of p);

B:= (point 8 of p);

C:= (point 6 of p);

q:=A--B;

vertical := .5[A,B]--C;

cima := buildcycle(q, subpath(4,8) of reverse p);

esquerda := buildcycle(subpath(4,6) of p, vertical, subpath(0,.5) of q);

direita := buildcycle(subpath(6,8) of p, subpath(0,.5) of reverse q, vertical);

draw p;

draw q;

draw vertical;

label(textext( \MPstring{labelcima} ), center cima);

label(textext( \MPstring{labelesquerda} ), center esquerda);

label(textext( \MPstring{labeldireita} ), center direita);

\stopuniqueMPgraphic

\starttext

\placefigure[center, nonumber]{}{

\Bolha[size=6cm]{$C_m$ (g/dm\high{3})}{mass (g)}{V\low{s} (dm\high{3})}

}

\startitemize[columns,two] %% try only \startitemize

\startluacode

flasks={20,25,50,100,200,250,500,1000}

k=1

repeat

Cm=math.random(1,10)/10

Vs=flasks[math.random(1,8)]/1000

mass=Cm*Vs

--incognita=math.random(1,3)

incognita=1

context.item()

if incognita==1 then

context("\\Bolha[size=4cm]{$C_m$}{%.2f g}{%.1f dm\\high{3}} mass=%.2f g", mass, Vs, mass)

elseif incognita==2 then

context("\\Bolha[size=5cm]{%.1f g/dm\\high{3}}{m}{%.1f dm\\high{3}}  mass= hidde", Cm, Vs, mass)

elseif incognita==3 then

context("\\Bolha[size=5cm]{%.1f g/dm\\high{3}}{%.2f g}{V\\low{s}}   mass=%.2f g", Cm ,mass, mass)

end 

k=k+1

until k==5

\stopluacode

\stopitemize

\stoptext

%%% stop code