Test cases like the following one produce different result sets if it's run
with and without th option --ps-protocol.

CREATE TABLE t1(a INT);
--enable_metadata
(SELECT MAX(a) FROM t1) UNION (SELECT MAX(a) FROM t1);
--disable_metadata
DROP TABLE t1;

Result sets differ in metadata for the query
  (SELECT MAX(a) FROM t1) UNION (SELECT MAX(a) FROM t1);

The reason for different content of query metadata is that for queries
with union the items being created on JOIN preparing phase is placed into
item_list from SELECT_LEX_UNIT whereas for queries without union item_list
from SELECT_LEX is used instead.