I have successfully used my host’s implementation of Let’s Encrypt for several domains. I tried just now on a new one and it has supplied a certificate for a not-trusted lokacije.ugostitelj.hr by COMODO RSA.
Can you provide the logs / screen dump or more info about where you saw the certificate ? This was a Comodo cert from what you say (so can’t have been issued by Let’s Encrypt). Since both domains seem to point to the same hosting provider, my guess is some sort of miss-configuration at your host