The issue is that adding the modal too soon after setting the main page fails on iOS. I cannot figure out how to tell if the MainPage is ready to add the modal. Right now I basically have to start a timer and keep trying to add it until it succeeds. A non-ideal solution.
You'll get this warning in your output log when it happens:
Warning: Attempt to present <Xamarin_Forms_Platform_iOS_ModalWrapper: 0x1647b8c0> on <Xamarin_Forms_Platform_iOS_PlatformRenderer: 0x16117c80> whose view is not in the window hierarchy!
Forms version 1.4.2