I'm not sure if this is a defect in 'mv_conffile' of the dpkg-maintscript-helper scripts.
dpkg-maintscript-helper handles conffiles.
But in this case the 'rules.v4' file of the package 'iptables-persistent' is a configuration file not a confile.
Quote from the Depian Policy 10.7 :
"Almost all conffiles are configuration files, but many configuration files are not conffiles."
A configuration file could be any file which customizes the behaviour of a program. But a conffile has to be shipped within the package and listed in a package's 'conffiles' file.
Neither the old 'rules' nor the new 'rules.v4' file are shipped within the package and so they are not a conffile.