This sounds like the password manager in your device/browser is asking if you wanted to update your stored password with the default pseudo password used in the form.
You had not changed the password form entry field, but you choose to allow your password manager to update.
Your password manager was not intelligent enough to notice that the form was pre-populated with a pseudo value. Your password manager only looked at what it had stored, and what the page was displaying, rather than looking to see if you actually changed values within the field.
By blindly clicking through your password manager's prompt and choosing "Yes. Please update the stored password.", it did exactly that. Your password manager updated the stored password, even though you never updated it with UBB.threads.
