Wrong warnings on 2-nd execution of PS for query with GROUP_CONCAT

SET optimizer_switch='outer_join_with_cache=off';

set group_concat_max_len=5;

create table t1 (a int, b varchar(20));

create table t2 (a int, c varchar(20));

insert into t1 values (1,"aaaaaaaaaa"),(2,"bbbbbbbbbb");

insert into t2 values (1,"cccccccccc"),(2,"dddddddddd");

prepare stmt1 from "select group_concat(t1.b,t2.c) from t1 left join t2 using(a) group by t1.a";

execute stmt1;

execute stmt1;

deallocate prepare stmt1;

drop table t1, t2;

set group_concat_max_len=default;

execute stmt1;

group_concat(t1.b,t2.c)

aaaaa

bbbbb

Warnings:

Warning	1260	Row 1 was cut by GROUP_CONCAT()

Warning	1260	Row 2 was cut by GROUP_CONCAT()

execute stmt1;

group_concat(t1.b,t2.c)

aaaaa

bbbbb

Warnings:

Warning	1260	Row 3 was cut by GROUP_CONCAT()

Warning	1260	Row 4 was cut by GROUP_CONCAT()

deallocate prepare stmt1;

execute stmt1;

group_concat(t1.b,t2.c)

aaaaa

bbbbb

Warnings:

Warning	1260	Row 1 was cut by GROUP_CONCAT()

Warning	1260	Row 2 was cut by GROUP_CONCAT()

execute stmt1;

group_concat(t1.b,t2.c)

aaaaa

bbbbb

Warnings:

Warning	1260	Row 1 was cut by GROUP_CONCAT()

Warning	1260	Row 2 was cut by GROUP_CONCAT()

deallocate prepare stmt1;