On my server (Debian Stretch), there are 2 vhosts :
only sub.example1.com has a Letsencrypt certificate (and it works).
sub.example2.com has no certificate
Nevertheless, checking sub.example2.com on a ssl tester (such as https://www.ssllabs.com/ssltest/analyze.html?d=s2.example.com) reveals that he tries to use the certificate of sub.example1.com (of course, it fails as the names mismatch).
Is it a normal behaviour or is it due to a misconfiguration ? Should I give more details ?