The following query looks correct according to a standard SQL understanding of JSON_ARRAYAGG():

select json_arrayagg(a order by a asc)

from (select 1 a union select 2 a) t

It doesn't get rejected by MariaDB 10.5.1's parser, but produces the wrong result

[2,2]

If this is not supported (it isn't according to the docs: https://mariadb.com/kb/en/json_arrayagg), then I would have expected a syntax error, instead of a wrong result.