Previously the common name of a san certificate was the first domain name mentioned in the ini file at domains = …
Since a few days/weeks(?) after adding a domain name and requesting a new certificate a random(?) domain name is shown for the common name. (Seems not alphabetically, but just one from the list… allbeit always the same.)
The directory in /etc/letsencrypt/live/ where the certificates are stored still has the name of the first domain name.
Seems that the client uses the first domain name to handle things… but the server creates a certificate with anothter common name.