Why can't I disable IPv6 with Informix on CentOS 7

Question

I've been trying to get Informix (Dynamic Server) (v12.10) working on a a CentOS 7 VM (provisioned using Vagrant), to accept IPv4 connections (from a JDBC application). The CentOS box version is v2004.01.

I've followed instructions to disable IPv6 binding using i) creating the empty file ($INFORMIXDIR/etc/IFX_DISABLE_IPV6), readably by the informix user, and... ii) setting the environment variable IFX_DISABLE_IPV6 (to the value yes) when starting the Informix server (using oninit).

I've restarted the Informix server but this just doesn't seem to work. The only way I can connect using IPv4 is to use a wild-carded host name in the sqlhosts file (or explore the DBSERVERALIASES route).

Strangely, the IFX_DISABLE_IPV6 method worked fine on older versions of the CentOS box that I have used (and these older versions did have IPv6 enabled hence I had to find the IFX_DISABLE_IPV6 method!).

Any ideas on what's going on would be greatly appreciated!