Clear cache,flush yet the login page doesn't show it.
Anyone any idea is this a bug?
If your theme is extending Magento_Blank theme, then you don't need to copy the entire contents of the customer_account_login.xml file to your theme to make changes to it --> this will "bug out" the CAPTCHA form.
The proper way to make changes is to use and to add, remove or move the elements as you want.
You can read instructions here on how to do that: http://devdocs.magento.com/guides/v2.2/frontend-dev-guide/layouts/xml-instructions.html
If you are facing the same issue, you can quickly test it by renaming/removing your theme's
customer_account_login.xml and clearing cache.
Then you can make the customizations the proper way based on the official documentation.