A number of significant things have been fixed in the git master branch since the 3.1.0 tarball, but all or some of those fixes have also been backported to the 3.1 branch.
I do not know if they have been generated into new Kamailio 3.1.0 tarballs in www.kamailio.org/pub per se, but daily builds from git are available from sip-router.org. For this reason, there is a more or less standing policy not to break the build, especially of maintenance branches. :-)