My general fix for this is to add autocomplete: discourse but I don’t think this is correct in this case.
This is actually not exactly the same as this broken chrome behavior: 587466 - chromium - An open-source project to help move the web forward. - Monorail
Somehow, some extension is adding text to new-account-challenge after we explicitly set it from here: https://github.com/discourse/discourse/blob/6de254f64263d7e3cc223404348c03e23c85d904/app/assets/javascripts/discourse/controllers/create-account.js.es6#L196-L199
This is clearly bad browser extension behavior to me. Why is it touching an INPUT after it already has text in it?
I am not sure there is anything we can do here short of isolate what browser extension is causing this and raising an issue with the authors of said extension.