The form action appears to use https. But yeah, it is 2016 and https should be the norm for even just loading pages with forms.
Of course I could just throw cloudflare in front of everything, but no thank you.
Shows working sni with nginx and let's encrypt. You've probably done the same config error
All I know is I didn't have to do anything special to get the 2nd domain running ssl on the same box. It just worked right away as expected. I wasn't even aware that there might be complications.