I restored a backup of the database into another one to check the tables. My result is kind of weird or (for me) at least not explaining how the problem evolved.
I fetched the groupid from the "client" table and compared them with the assigned maildomains in the table "mail_domain" and its corresponding sys_groupid. From this point everything was fine, meaning each domain was linked to its corresponing customer.
When checking the table "mail_forward" and its "catchall" entries, both catchall entries belong to only one customer / sys_groupid, eventhough by that time one of the affected domains was not linked to the customer at all (see check above).
@till: I do not have a table mail_catchall, the catchall entries are in table "mail_forwarding" with field "type" set as "catchall". (Version: 18.104.22.168)
Unfortunately my binlog data is already cut off so I can not check which sql statements may have produced this issue which I could not reproduce yet, e.g. by switching mail_domains accross cuostmers with existing catchall entries.
Any other logs I coul check? If not I'd just close this topic and keep an eye on it.