ORDER BY on SAI field not supported for single-partition reads

In the example table below an SAI index is used for the item_category field. The item_name column is the clustering key.

CREATE TABLE shopping_cart(
    id bigint,
    item_name text,
    item_category text,
    PRIMARY KEY (id, item_name)
) WITH CLUSTERING ORDER BY (item_name ASC);

CREATE INDEX shopping_cart_item_category_idx
ON shopping_cart (item_category)
USING 'sai';

An execution of a query such as

SELECT id, item_name, item_category 
    FROM shopping_cart 
    WHERE id = 12345                     -- partition key
    AND item_category = 'softdrinks'     -- 2ndary index

is successful. This query selects all softdrink items of the partition. The items in the result set are sorted according to the clustering key. See Query Result

However, the use of an ORDER BY clause in the query such as

SELECT id, item_name, item_category 
    FROM shopping_cart  
    WHERE id = 12345                     -- partition key
    AND item_category = 'softdrinks'     -- 2ndary index
    ORDER BY item_name ASC               -- clustering key

fails with the error message ORDER BY with 2ndary indexes is not supported, except for ANN queries. Since this query reads only a single partition, the query with ORDER BY should be allowed in my opinion.

The fact that a select with ORDER BY fails on multiple partitions, as shown below, is understandable to me as Cassandra does not support global SAI sorting, currently.

SELECT id, item_name, item_category 
    FROM shopping_cart 
    WHERE item_category = 'softdrinks'   -- 2ndary index
    ORDER BY item_name ASC               -- clustering key

Is the observed behavior merely an issue of the query parser?