When the plugin notifies Stash using the Pipeline step syntax, it isn't including the branch name so a build result for one branch is marked against all branches.
Eg:

	step([$class: 'StashNotifier',  credentialsId: 'mycredrentials', stashServerBaseUrl: 'http://mystash'])

Jenkins version 2.7.4
Stash Notifier Plugin version 1.11.4