When setting the new threshold to 1, if there are new warnings, the plugin marks the build as failure. If the next build will not have any new warnings (even without any change to any file) the status will be change back because the last new warnings are not consider new anymore.
There is another option to set the total threshold. The problem with this way is that whenever a warning is fixed, someone will need to change this threshold manually.
Comparing the total number of warnings with the total number of warnings in the last stable/unstable build might not be enough because someone can commit a change which will fix a warning but will add a new one.