you have added mydestination = mydomain.example and so the valid users for mydomain.example will be searched in local_recipient_maps. It is very clear from the logline that local delivery agent is selected to deliver the mail, as local domain class matched. See postfix address classes for more help
Modify your main.cf like below
#/etc/postfix/main.cf
mydestination =
# set to localhost localhost.$mydomain or remove mydomain.example for now
# Ensure that postmap -q mydomain.example mysql:/etc/postfix/mysql-virtual-mailbox-domains.cf is returning mydomain.example and
# postmap -q myuser@mydomain.example mysql:/etc/postfix/mysql-virtual-mailbox-maps.cf succeeds