UnityEngine.UI.Dropdown broken on iOS? Unity 5.5

Crossposting from the UI forum since this is iOS-specific:

I have an app that runs fine on desktop, but when I run it on iOS, the Dropdown UI component specifically doesn't work - clicking on it produces the following error, and it fails to render correctly (Unity 5.5, iOS 9.x, Xcode 8.2.1).

Any ideas how to fix or circumvent this? It looks like a problem with one of the Unity built-in UI structs: UnityEngine.UI.Navigation which is used by the Dropdown component...

Full error stack...

UnityEngine.UI.Dropdown broken on iOS? Unity 5.5