Although the CASE WHEN clause returns the same value as the direct concatenation, FIELD yields different results in each case.

select FIELD((case when true then ('i' || 'o') else '0' end), 'c') as c0;

+----+

| c0 |

+----+

|  0 |

+----+

1 row in set, 2 warnings (0.00 sec)

select FIELD(('i' || 'o'), 'c') as c0;

+----+

| c0 |

+----+

|  1 |

+----+

1 row in set, 3 warnings (0.00 sec)