The CAPTCHA field on the login screen is not reachable using the tab key

One the login screen, it seems impossible to navigate to the CAPTCHA "Answer" field using the keyboard tab key.

Expected: Tab from the password field should go to the answer field, and then navigate through elements of the captcha (links, etc)
Actual: Tab from the password field goes to the submit button, (when it will obviously not work if the CAPTCHA answer field has not been filled)