The cause of the bug is that Item_func_in::fix_fields did not fully update its
used_table_cache. This was the cause for not_null_tables in setup_conds() to be
still 0 after the call
  not_null_tables= (*conds)->not_null_tables();
As a result the condition in setup_conds()
  if ( ... (table->table->map & not_null_tables) ...)
failed, which was the cause for the ON expression not to be added to conds, and later
the optimizer couldn't detect that it could apply the OUTER JOIN ==> JOIN optimization.