Hi,
Good catch, that was an oversight on our end. The password field on the checkout login form has been rendering with a placeholder since we switched it to the Joomla 4+ password form layout (which carries its own hint, displayed as a placeholder), and the username input next to it was left as a plain text input without a matching placeholder attribute, hence the visual inconsistency you noticed.
We just published a new build of HikaShop 6.4.1 with the fix. The username field now uses the same placeholder as its label and matches the password field's behaviour. Please download the latest 6.4.1 from your member area and reinstall it over your current install.