The developer has to add a formkey for the new export option because the formkey is required for security reasons. Without the formkey, the request will be rejected and redirected to the dashboard page. Verified References: [Magento 2.4 User Guide] [Magento 2.4 DevDocs]

When adding custom export options to grids in Magento, it's crucial to include a form key for actions that involve form submission. Magento relies on form keys for CSRF (Cross-Site Request Forgery) protection, so omitting the form key can lead to redirects or failed operations.

Form Key Requirement:

In Magento, the form key is a hidden token included in forms to ensure that the request is valid. This is particularly important for actions that change the state or export data, as it helps prevent unauthorized actions.

Adding a custom export button triggers a form submission, which requires a valid form key. Without it, Magento may redirect to a default page or the admin dashboard as a security measure.

Why Option C is Correct:

Option C correctly identifies the lack of a form key as the issue. When Magento detects a missing form key in sensitive operations, it defaults to a redirect to maintain security.

Option A, concerning the URI, is less likely to cause a redirection. Similarly, Option B regarding layout cache would not directly impact CSRF validation, which is the cause of the redirection here.

Solution:

Modify the button or form submission logic to include a form key, typically by adding form_key={{formKey}} in the URL parameters or within the form data.