Analysis:
The wrong result is a consquence of sorting the subquery
result and then selecting only the first row due to the
artificial LIMIT 1 introduced by the fix_fields phase.
Normally, if there is an ORDER BY in a subquery, the ORDER
is removed (Item_in_subselect::select_in_like_transformer),
however if a GROUP BY is transformed into ORDER, this happens
later, after the removal of the ORDER clause of subqueries, so
we end up with a subquery with an ORDER clause, and an artificially
added LIMIT 1.

The reason why the same works in the main 5.3 without MWL#89, is
that the 5.3 performs all subquery transformations, including
IN->EXISTS before JOIN::optimize(). The beginning of JOIN::optimize
does:
  if (having || (select_options & OPTION_FOUND_ROWS))
    select_limit= HA_POS_ERROR;
which sets the limit back to infinity, thus 5.3 sorts the whole
subquery result, and IN performs the lookup into all subquery result
rows.

Solution:
Sorting of subqueries without LIMIT is meaningless. Since LIMIT in
subqueries is not supported, the patch removes sorting by setting
  join->skip_sort_order= true
for each subquery JOIN object. This improves a number of execution
plans to not perform unnecessary sorting at all.