Unclear SQLFLUFF lint violation: AM06 "ambiguous column reference": 'GROUP BY/ORDER BY' error

Question

In this code:

SELECT
    kgi.ingredient_id,
    STRING_AGG(val, ', ') AS aka
FROM {{ ref('stg_bq_kg__ingredients') }} AS kgi,
    UNNEST(synonyms) AS synonym,
    UNNEST(synonym.values) AS val
GROUP BY kgi.ingredient_id

I get L: 7 | P: 1 | AM06 | Inconsistent column references in 'GROUP BY/ORDER BY' clauses. | [ambiguous.column_references] but I am not quite sure why since the column is being referenced clearly.

Note I am using dbt_templater in my .sqlfluff.