I'm testing pre and post scripts for a single app today and using selective sync to publish just that app, I've done this several times today but each time I remain in the UI after tagging the app for a selective sync and then going to the sync page to publish.
I just accidentally ran a full sync (against our best practices during business hours) because I had tagged the app, clicked save and close, reopened the publisher and went to the sync page and clicked the publish button. It gave me the message that it was now running instead of the selection list to pick the apps for selective sync.
Either persist the selective sync flags on an app/update or change the behavior of the publish button so that if nothing is tagged for selective sync, it asks are you sure if you want to publish all updates/apps.