Configure a default source route for IPv6
When new IPv6 addresses are allocated in cPanel, the default source / outgoing route for IPv6 connections is the last IPv6 address to be added. This means that it will change throughout time and is a nightmare when trying to configure a firewall on the other side of the connection.
cPanel's networking setup is not CentOS standard, so there is no reasonable workaround to set a default source route for IPv6.
I suggest that cPanel alter their network configuration scripts for IPv6 to set preferred_lft to zero on all IPv6 addresses bound apart from the first one. That way, the source IPv6 address will be static.