Sonar throwing the issues on the older code even though the quality gates threshold property is "new code" only


we are running sonar community edition version-7.7.
we are scanning a repository (java code) for vulnerabilities from nearly 4 months.
Sonar bug report throwing one issue and says it is 15days older commit, but jenkin has 4-5 green builds post that, because we are failing the builds if issues count is greater than one on new code. the new code period by default is the snapshot version.
Point to note is Issue thrown is valid and SonarQube captures delta from previous snapshot version to current even if the code is 15days old.
the question is :roll_eyes:
Why Sonar dint throw issue earlier is something to worry about.