Meta query for a custom taxonomy on attachment

I have a custom taxonomy “length” with 3 values [‘long’, ‘medium’, ‘short’]
I have attached this taxonomy to attachments (using CPT UI)

Upon viewing the taxonomy I can see that I have successfully added ‘long’ to an image as it shows “count = 1”

However my query is returning no records.

Array
(
    [post_type] => attachment
    [post_mime_type] => image
    [post_status] => inherit
    [posts_per_page] => -1
    [meta_query] => Array
    (
        [0] => Array
            (
                [key] => length
                [value] => Array
                    (
                        [0] => 26
                    )
                [compare] => IN
            )

    )

)

If I remove the meta_query it does return all images.
I have confirmed that the ID for the ‘long’ taxonomy record is 26.
I can see in the wp_postmeta that the values exist for the correct records.

the request query is:

SELECT   wp_posts.* FROM wp_posts  INNER JOIN wp_postmeta ON ( wp_posts.ID = wp_postmeta.post_id ) WHERE 1=1  AND (wp_posts.post_mime_type LIKE 'image/%')  AND ( 
( wp_postmeta.meta_key = 'length' AND CAST(wp_postmeta.meta_value AS CHAR) IN ('26') )
) AND wp_posts.post_type = 'attachment' AND ((wp_posts.post_status = 'inherit')) GROUP BY wp_posts.ID ORDER BY wp_posts.post_date DESC 

I can see the problem is that the meta_value is stored as a serialized object and that:

 wp_postmeta.meta_key = 'length' AND CAST(wp_postmeta.meta_value AS CHAR) IN ('26')

will never be true.
I can write a custom sql query to fix this, but, I am wondering if there is something I am missing with the meta query?

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