Probably a bug I suppose. Arrays probably don't save them either I'd imagine with the .asJson expression. It doesn't mean the variables aren't saved with the save and load actions (can't look at the source to check right now).
Actually I can also see why it might not be a bug. As it is now any .asJson of a dictionary can be loaded into any other dictionary. If it saved instance variables too it would be like Sprite types where I assume you can only load a json string saved from the same type.