This is the missing image in post #1. It shows what occurs when we go to the options page where the Username input is being used. The Email to usernames field is empty, but the browser fills in the field assuming it's a log in.
Setting the input to search (or anything really) and hoping the browser will infer that you don't want to auto-complete it isn't the right way to do it. Assuming you are using <xf:tokeninput> or <xf:tokeninputrow>, what you are looking for is the autocomplete="off" attribute on either of those tags. That will end up in the generated input tag and is the way to explicitly tell the browser to not auto-complete or auto-select anything in that input field.
The HTML autocomplete attribute lets web developers specify what if any permission the user agent has to provide automated assistance in filling out form field values, as well as guidance to the browser as to the type of information expected in the field.
The browser is not permitted to automatically enter or select a value for this field.