This is a browser setting and your browser pre-fills the values when you open the settings page. Websites/applications used to have the power to disable this but more and more browsers override this behavior by default.
If you clear the settings in the administration area and then save the form, TestRail will correctly save your settings. It’s just that your browser pre-fills the information as soon as you open the settings page again and this will then look as if TestRail hasn’t cleared the fields.
The relevant keywords to disable this as part of your browser would be autofill or autocomplete (if you would like to disable this).
Please let me know in case anything is unclear and I look forward to your reply!