ERROR 1222 for equal-width nested set operations inside EXISTS

MariaDB Server 12.3.2 reports that SELECT statements have different column counts when a two-column UNION ALL has a parenthesized INTERSECT operand inside EXISTS. Every SELECT actually has two columns.

The set expression executes successfully outside EXISTS. A flat two-column UNION ALL and a nested single-column version also succeed inside EXISTS.

SELECT 1 AS result

WHERE EXISTS (

    SELECT 1, 2

    UNION ALL

    (SELECT 3, 4 INTERSECT SELECT 5, 6)

);

ERROR 1222 (21000): The used SELECT statements have a different number of columns