Where in list prepared statement only returns first of list

I have the following code. req.query.tags is a string of ids seperated by a comma, e.g. 1,2,3.

const query =
    'SELECT DISTINCT images.* FROM images LEFT JOIN images_has_tags ON image_id = images.id WHERE tag_id IN (?)';
const [rows] = await conn.query(query, [req.query.tags]);

This returns only one row, this row matches the first item in the list.
(If req.query.tags is 1,2,3, the query equals ...WHERE tag_id = 1)

Whereas

const query =
    'SELECT DISTINCT images.* FROM images LEFT JOIN images_has_tags ON image_id = images.id WHERE tag_id IN (1,2,3)';
const [rows] = await conn.query(query);

works fine.

Start at the conn.query call shown in the report and trace how its parameter array is handled for the IN (?) placeholder. Reproduce the behavior with tags=1,2,3 and compare it with the working literal query. Done means the intended handling is clarified through a tested driver behavior or documented usage.