The problem is that the svn-tag plugin takes the environment variables from the ancestor build and not the actual build that was triggered.
The issue can be solved if the environment variables from the actual build are put in the var map and then the ancestor build adds/overrides its environment variables to the map. That would not change the original behaviour of the svn-tag plugin but add the wanted functionality for it to work well together with the promoted build plugin.
The bugfix would be in the SvnTagPlugin.java in line 100.
EnvVars envVars = rootBuild.getEnvironment(buildListener);
EnvVars envVars = abstractBuild.getEnvironment(buildListener);
EnvVars rootVars = rootBuild.getEnvironment(buildListener);