MySQL Select Where Unexpected Results

Question

I have a Joomla (3.9.13) database that contains a table called fields_values, it is a denormalized key value store (so i've been told). It contains the following data;

+----------+---------+--------+
| field_id | item_id | value  |
+----------+---------+--------+
| 6        | 119     | bloggs |
| 17       | 119     | 1      |
| 19       | 119     | 45     |
| 21       | 119     | 55     |
| 17       | 119     | 14     |
| 17       | 119     | 100    |
+----------+---------+--------+

When I run this query;

SELECT `field_id`
     , `item_id`
     , `value` 
  FROM `fields_values` 
 WHERE `field_id` IN (6,17) AND `item_id` = '119'

I get these results;

+----------+---------+--------+
| field_id | item_id | value  |
+----------+---------+--------+
| 6        | 119     | bloggs |
| 17       | 119     | 1      |
| 17       | 119     | 2      |
| 17       | 119     | 13     |
| 17       | 119     | 14     |
| 17       | 119     | 100    |
+----------+---------+--------+

That all seems fine, but when I run this query (omit the 6 from IN clause);

SELECT `field_id`
     , `item_id`
     , `value` 
  FROM `fields_values` 
 WHERE `field_id` IN (17) AND `item_id` = '119'

I get zero results.

I would have expected the following results;

+----------+---------+--------+
| field_id | item_id | value  |
+----------+---------+--------+
| 17       | 119     | 1      |
| 17       | 119     | 2      |
| 17       | 119     | 13     |
| 17       | 119     | 14     |
| 17       | 119     | 100    |
+----------+---------+--------+

I've also tried this simple version of the query;

SELECT 
    `field_id`, 
    `item_id`, 
    `value` 
FROM 
    `o3m0z_fields_values` 
WHERE
    `field_id` = 17 
AND 
    `item_id` = '119'

I get zero results.

Here's the table information taken from HeidiSQL;

I've tried to replicate this on a fresh Joomla install and I can't. Why is this happening?

Note: I can't change the query as it's generated by the core Joomla system.