- [when doing a measurement after updating the version of the java plugin in the issue for the same rule shows two different severities]
Welcome to the community!
First, 5.6 is far beyond EOL. You should upgrade immediately. Your upgrade path is:
5.6 -> 6.7.7 -> 7.9.5 -> 8.5.1 (last step optional)
Second, yes this happens. We iteratively improve analysis. Sometimes that means we change the severity or type of a rule. New issues are raised with the new values but old issues are not updated.