Looked it up in the source, and the version is indeed hardcoded in the runtime :) Probably the way to detect the runtime version has changed somewhere in the past. I'm gonna investigate it.
Update
This is a bug in Mendix 2.5.* and has been reported. A workaround is to replace the getRuntimeVersion() method in javasource/communitycommons/Misc.java with
public static String getRuntimeVersion()
{
Package currentPackage = MxRuntime.class.getPackage();
String specVersion = currentPackage.getSpecificationVersion();
String implVersion = currentPackage.getImplementationVersion();
if(specVersion != null && implVersion != null)
return implVersion + " (build " + specVersion + ")";
return
"unreleased";
}