When I use flushright, my Japanese characters get widely spaced as if flushing right doesn't work

Minimal example:

% ===============================

 \definefallbackfamily
  [archimate]
  [ss]
  [Hiragino Sans]
  [preset=range:japanese,
   tf=style:W3,
   it=style:W3,
   bf=style:W5,
   bi=style:W5,
   force=yes]

\definefontfamily [archimate] [ss] [Optima]

\setupbodyfont[archimate]

\startsetups [japanese]
  \setscript [nihongo]
\stopsetups
\setuplanguage [ja] [setups=japanese]
\mainlanguage [ja]

\starttext
\setupindenting[none]
\setupindenting[no]
\framed[width=\textwidth,frame=off,align=flushleft]{作図：A bit of text}
\framed[width=\textwidth,frame=off,align=flushright]{作図：A bit of text}
\framed[width=\textwidth,frame=off,align=flushright]{作図: A bit of text}
\framed[width=\textwidth,frame=off]{作図: A bit of text}
\stoptext
% ===============================