Hi,
If the viewer has a pending update (i.e. the "update available" indicator is showing) and the reader re-enters the collection we will automatically apply the update. Otherwise the reader has to tap the update available indicator. We designed it this way to ensure your readers don't lose context for where they are when an update is applied to a collection they are actively viewing.
Android and iOS should have the same update model. If you are seeing different behavior between the two viewers please share exact steps to reproduce the behavior so we can investigate.
Thanks,
Neil