How long did you wait to visit the site after adding it to pangolin? I ask, because if you set it up and go within 30 seconds of saving you may see the cert warning initially. It can LetsEncrypt time to verify a site and issue a cert. it’s usually pretty fast, but I’ve seen it take up to 30 seconds or a little longer.

I’ve hit the same thing where I setup a site through Pangolin and go to it too fast, but if I just close the tab and wait about 20 seconds then go again, it all comes up with a proper cert. hope that helps.