It seems to me a bug with a simple case and a subquery:

Example:

--works fine (searched case with a subquery)
SELECT
  CASE
    WHEN (SELECT 'A' FROM RDB$DATABASE) = 'A' THEN
      'Y'
    WHEN (SELECT 'A' FROM RDB$DATABASE) = 'B' THEN
      'B'
    ELSE
      'N'
  END
FROM RDB$DATABASE

--works fine (simple case without a subquery)
SELECT
  CASE 'A'
    WHEN 'A' THEN
      'Y'
    WHEN 'B' THEN
      'N'
    ELSE
      'U'
    END
FROM RDB$DATABASE

--don't work (simple case with a subquery)
SELECT
  CASE (SELECT 'A' FROM RDB$DATABASE)
    WHEN 'A' THEN
      'Y'
    WHEN 'B' THEN
      'N'
    ELSE
      'U'
   END
FROM RDB$DATABASE

/*
Invalid token.
invalid request BLR at offset 110.
context already in use (BLR error).
*/