Thats the normal behaviour of the apache webserver. When you use the IP address to access a server and no website with that domain exists, then apache will deliver the first vhost in alphabetical order that it finds for this IP.
It might be that you got a different website before the update as ISPConfig uses now a prefix for the vhost files. To show the website again that was shown before the ispconfig update, edit that website in ispconfig and change a value, e.g. quota and hit save, so that ispconfig reconfigures the vhost file for that website too.