10.1 disallowed use of non-parenthesized UNION parts that have ORDER or LIMIT clauses, according to the SQL Standard. That was a merge from MySQL-5.6.

This script:

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a INT);

INSERT INTO t1 VALUES (10),(10);

SELECT * FROM t1 LIMIT 1

UNION

SELECT * FROM t1 LIMIT 1;

returns an error starting from 10.1:

ERROR 1221 (HY000): Incorrect usage of UNION and LIMIT

However, if I use LIMIT in combination with ROWS EXAMINED:

SELECT * FROM t1 LIMIT ROWS EXAMINED 1

UNION

SELECT * FROM t1 LIMIT ROWS EXAMINED 1;

it returns a result instead of an error:

+------+

| a    |

+------+

|   10 |

+------+

1 row in set, 1 warning (0.00 sec)

It should be fixed to require parentheses as well, like in case of a "normal" LIMIT clause.

Most likely, LIMIT ROWS EXAMINED was just forgotten during the merge.

The documentation at:
https://mariadb.com/kb/en/mariadb/union/
says:

Individual selects can contain their own ORDER BY and LIMIT clauses. In that case, the individual queries need to be wrapped between parenthesis.