If you set WAGTAIL_2FA_REQUIRED = True, the styling will be missing on the otp_form.html when you try to loggin.
This happend because when the page tries to get the css file at: .../static/wagtailadmin/css/core.css
It redirects to the otp token (otp_form.html) page.
I'm using Wagtail 4.2.4