works. 

But

\starttext

\startMPcode
  label("\m{\root3\of2}", origin);
\stopMPcode

\stoptext

doesn't.

Running `LuaMetaTeX' on `trial' with ``/home/fvleung/ConTeXt/tex/texmf-linux-64/bin/mtxrun --autogenerate --script context trial.tex''

resolvers       | formats | executing runner 'run luametatex format': /home/fvleung/ConTeXt/tex/texmf-linux-64/bin/luametatex --jobname="trial" --fmt=/home/fvleung/ConTeXt/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luametatex/cont-en.fmt --lua=/home/fvleung/ConTeXt/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luametatex/cont-en.lui cont-yes.mkiv --c:currentrun=1 --c:fulljobname="./trial.tex" --c:input="./trial.tex" --c:kindofrun=1 --c:maxnofruns=9 --c:texmfbinpath="/home/fvleung/ConTeXt/tex/texmf-linux-64/bin"
open source     > level 1, order 1, name 'cont-yes.mkiv'
fonts           > beware: no fonts are loaded yet, using 'lm mono' in box
system          >
system          > ConTeXt  ver: 2021.07.27 19:27 LMTX  fmt: 2021.7.28  int: english/english
system          >
system          > 'cont-new.mkxl' loaded
open source     > level 2, order 2, name '/home/fvleung/ConTeXt/tex/texmf-context/tex/context/base/mkxl/cont-new.mkxl'
system          > beware: some patches loaded from cont-new.mkiv
close source    > level 2, order 2, name '/home/fvleung/ConTeXt/tex/texmf-context/tex/context/base/mkxl/cont-new.mkxl'
system          > files > jobname './trial', input './trial.tex', result './trial'
fonts           > latin modern fonts are not preloaded
languages       > language 'en' is active
open source     > level 2, order 3, name './trial.tex'
fonts           > preloading latin modern fonts (second stage)
fonts           > 'fallback modern-designsize rm 12pt' is loaded
metapost        > initializing instance 'metafun:1' using format 'metafun' and method 'default'
metapost        > loading 'metafun' as 'metafun.mpxl' using method 'default'
metapost        > initializing number mode 'scaled'
metapost        > trace > This is MPLIB for LuaMetaTeX, version 3.11, running in scaled mode.
metapost        > trace >
metapost        > trace > loading metafun for lmtx, including the plain 1.004 base definitions
metapost        > trace >
tex error       > tex error on line 7 in file ./trial.tex: Argument of \root has an extra }

32\normalstopimath \fi

<inserted text>
    \par
<to be read again>
    }
<lua output> }
   

<local> \global \setbox \mptextbox \hbox {\clf_mptexttoks
    }

<inserted text>
    [[special cmd: end local call]]

<to be read again>
    \relax
...


<line 4.5> \stopMPcode
   
1     \starttext
2    
3     \startMPcode
4       label("\m{\root3\of2}", origin);
5     \stopMPcode
6    
7 >>  \stoptext
8    
I've run across a '}' that doesn't seem to match anything. For example,
'\def\a#1{...}' and '\a}' would produce this error. The '\par' that I've just
inserted will cause me to report a runaway argument that might be the root of the
problem.
mtx-context     | fatal error: return code: 256

TeX Output exited abnormally with code 1 at Fri Sep  3 17:44:19