Invalid SQL statement selection and invalid syntax errors when `THEN` is used

[SanjulaGanepola]

Attempting to run the following statement fails as the selection range becomes invalid

CREATE MASK mask_ccnum
  ON customers
  FOR COLUMN cc_number
  RETURN
    CASE
      WHEN cc_number IS NULL THEN NULL
      ELSE
        'XXXX-XXXX-XXXX-' || SUBSTR(cc_number, CHAR_LENGTH(cc_number) - 3, 4)
    END
  ENABLE;

[SanjulaGanepola]

Could be an issue with how THEN is handled.

More syntax errors found by @forstie:

--  Generate SQL
--  Version:                   	V7R4M0 190621
--  Generated on:              	09/29/25 15:26:10
--  Relational Database:       	IHOST
--  Standards Option:          	Db2 for i
CREATE MASK tims_big_bank.mask_security_question_answer_on_customers ON tims_big_bank.customers AS c
FOR COLUMN customer_security_question_answer RETURN
CASE
  WHEN
    qsys2.VERIFY_GROUP_FOR_USER(session_user, 'ADMIN_G99') = 1
    THEN c.customer_security_question_answer
  WHEN
    qsys2.VERIFY_GROUP_FOR_USER(session_user, 'CUST_G99') = 1
    THEN c.customer_security_question_answer
  ELSE '*****'
END ENABLE;