Remove the “www” and the HTTPS encryption will work just fine.
Technically speaking, the encryption itself works just fine with wrong domain too, but as it's not listed on certificate it's not valid for www.sopuli.xyz domain.
Some time after going to sopuli.xyz on Firefox on my laptop and sharing the link to my wife the www was injected. I copied the address in Firefox’s address bar and pasted that into telegram. Which she then clicked that link on her phone.
The "www" in a URL isn't just random. It either isn't part of a websites address, or it is. Whether it works, or redirects to something that works, is up to any given website.
Some don't work at all if you add it when it's not supposed to be there.
That's a good thing but I know that Firefox has a proceed anyway button. Which Safari didn't have when I encountered the issue. (Maybe it's a different config)