[NTG-context] strange behavior with \groupedcommand in recent versions of LMTX

[mf]

Hello list,
this is a MWE that worked right in MkIV and in LMTX versions (at least) 
until April 22nd, 2020.

There's a macro, \wordorparagraphright, that takes one argument, makes a 
hbox with it and measures its width

- if its width is less than \hsize, then it calls \wordright

- if its width is more than \hsize, then it ends the current paragraph 
and starts a right-aligned paragraph with the argument

There are two implementations in the MWE: one is through 
\groupedcommand, and that's the one that worked once and doesn't now.

----------------------------------------------------
\unexpanded\def\groupedwordorparagraphright{\dosingleempty\dogroupedwordorparagraphright}
\def\dogroupedwordorparagraphright[#1]{\groupedcommand{\setbox\scratchbox\hbox\bgroup}
{\egroup\ifdim\wd\scratchbox < \hsize
     \wordright{\unhbox\scratchbox}
   \else
     \par\penalty10000
     \startalignment[flushright]\unhbox\scratchbox\stopalignment
   \fi}}

\unexpanded\def\wordorparagraphright#1{%
   \setbox\scratchbox\hbox{#1}
   \ifdim\wd\scratchbox < \hsize
     \wordright{#1}
   \else
     \par\penalty10000
     \startalignment[flushright]\unhbox\scratchbox\stopalignment
   \fi}

\starttext
   \type{\wordorparagraphright} should use \type{\wordright} if its argument
   is narrower than \type{\hsize}, or make a right-aligned paragraph with it
   when it's wider.

   Below, \type{\wordorparagraphright} is tested with a short text, that
   has enough room on the same line.
   \blank

   Reference:

   \input ward\wordright{same line, right}

   \blank

   Implementation as a grouped command (it doesn't work):

   \input ward\groupedwordorparagraphright{same line, right}

   \blank

   Normal implementation, not as a grouped command (it works):

   \input ward\wordorparagraphright{same line, right}
\stoptext
----------------------------------------------------

Unfortunately the grouped version is the one I need, because it's used 
in a register, and it would give an error if not defined through 
\groupedcommand:

\setupregister[myregister]
   [pageleft=\wordorparagraphright\bgroup,pageright=\egroup]

TIA,
Massi