When adding a new threshold package in threshd-configuration.xml, there is a way to reload the configuration of this particular file. But the current code, should also reload thresholds.xml because those files are tight to each other. Otherwise, the new change will be ignored, specially when displaying the list of configured groups in the WebUI.
So, the correct behavior is, after changing just a threshold definition in thresholds.xml, only that file should be reloaded. But, after changing threshd-configuration.xml, this file and thresholds.xml should be reloaded in order to properly apply the changes.
The test called testReloadThreshdConfig defined in ThresholdingVisitorTest also confirms this. The test is working because both files are reloaded, so the problem is handling the reload operation for thresholds configuration in Collectd.handleReloadDaemonConfig.