SELECT CONCAT(<cols not in GROUP BY list>) throws error: MCS-2021

SELECT CONCAT(<cols not in GROUP BY list>) throws error: MCS-2021.

Repro
------
CREATE TABLE t1 (
cid mediumint(9) NOT NULL,
firstname varchar(32) DEFAULT '' NOT NULL,
surname varchar(32) DEFAULT '' NOT NULL
) ENGINE=columnstore;
INSERT INTO t1 VALUES (1,'That','Guy');
INSERT INTO t1 VALUES (2,'Another','Gent');

MariaDB [testMcs]> SELECT cid, CONCAT(firstname, ' ', surname) AS fullname, COUNT FROM t1 GROUP BY cid;
ERROR 1815 (HY000): Internal error: MCS-2021: 'fullname' is not in GROUP BY clause. All non-aggregate columns in the SELECT and ORDER BY clause must be included in the GROUP BY clause.

But SELECT without CONCAT() works.
MariaDB [testMcs]> SELECT cid, firstname, surname, COUNT FROM t1 GROUP BY cid;
-----------------------------+

cid

firstname

surname

COUNT

-----------------------------+

-----------------------------+
2 rows in set (0.055 sec)

Expected behavior:
SELECT CONCAT(<cols not in GROUP BY list>) should work.