You aren’t using the activity correctly, although it is badly designed (I dunno why it takes an iCollection<Object> rather than an iCollection or iCollection<T> as it creates issues with certain collection types right now).
The application care is also confusing because the ‘Collection’ property is an in only, you need to use the ‘Result’ property to get the collection back out with the added item(s).
It should just be an in/out property. I have skipped it completely and continued to use Invoke Method to add values to a collection.