problem

1. Open a task liveCheckOnEditing = 0
2. Change to a task where liveCheckOnEditing = 1
3. The spellcheck will still not work

reason

The reason for this behaviour is a logical bug in the spellcheck controller: 

/***
 * After task config load event handler.
 */
onEditorConfigLoaded:function(app, task){
    var me=this,
        isPluginActive = app.getTaskConfig('plugins.SpellCheck.liveCheckOnEditing');
    me.setActive(isPluginActive);
},

1. The above event handler is evaluating the tasks config.
2. If liveCheckOnEditing in a task is disabled, this disables the whole controller
3. After changing to another task - where liveCheckOnEditing is enabled - the event handler onEditorConfigLoaded is never called again since the whole controller is deactivated

Conclusion

deactivating of a controller is a useful way to disable a controller / feature but can not be controlled from inside the controller.

Solution

spellcheck disabling must be implemented via an internal deactivation flag instead of deactivating the whole controller