I'm not sure I know enough either way to say - can you send the project that exhibits the problem? (Also, please send your workaround project). Thanks!
Due to the nature of how viewport actions work, there's not a "real" event; in fact even calling window.event will not give a defined one.
However, in this and a lot of other contexts, we generate "faux" events that usually have some of the fields filled in to give better context.
In fact, if you use the HYPE_eventListeners for the HypeEnterViewport or HypeExitViewport, you will get this faux event filled in. It does seem like a long standing bug that we don't use also put it in the javascript action handler.
I should mention - the faux event only has a type field (of either HypeEnterViewport or HypeExitViewport). If there's anything more you need it won't be there, and you'd need to get it another way.