Why does my old e-mail appear in the sign in section when i log out?

Does your browser have an auto-fill feature turned on or an ad-on that saves login info? If it does, it may be adding the email address itself. 

Not even, and it managed to put my username as my old email, something is wrong here

In order to help you with this situation, we believe your best option is to contact our telephone support team for further assistance, as they have the option to view your screen to help resolve the issue. To contact us please follow this link: Contact Us form. 

​​​​​​​Thank you for choosing TurboTax.