MaxScale masking does not work with UNION ALL

mysql> SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT COL FROM TABLE LIMIT 2;

+------+

| 1 |

+------+

| 1 |

| 2 |

+------+

2 rows in set (0.00 sec)

mysql> SELECT 1 UNION SELECT 2 UNION SELECT COL FROM TABLE;

ERROR 1141 (HY000): The field COL that should be masked for 'user'@'host' is used in the second or subsequent SELECT of a UNION, access is denied.

[Mask-Strings]

type=filter

module=masking

large_payload=abort

rules=/etc/mask-rules.json

[Mask-Service]

type=service

filters=Mask-Strings

router=readwritesplit

servers=server1,server2

user=MAX_SCALE_USER

password=MAX_SCALE_PASSWORD

enable_root_user=false

{

      "replace": {

        "column": "COL"

      },

      "with": {

        "value": ".",

        "fill": "."

      }