When rendering {{ routes.storefront_login_url }} within the Section Rendering API - the return_to parameter is set to include the the sections query parameter.
I can’t see a world where a user would ever need to be returned to an API response, and could lead to inconsistent behaviour should a section ever be re-rendered via the Section Rendering API.
Test case: https://10sq-plus-sandboxes.myshopify.com/?sections=sr-api-test