As I have lost a few hours with this issue, I post this information here.
In my App, I have some BoxViews (used as "Lines").
I have to make the BoxViews visible and invisible.
Unfortunately this works in Android and WP but not in iOS.
I then have tried to add an additional StackLayout, add the BoxView to the StackLayout and make the StackLayout visible/invisible.
Unfortunately this also don't work in iOS.
This is a known (and confirmed) bug:
Link: https://bugzilla.xamarin.com/process_bug.cgi
Workaround:
Use Frame instead Boxview - Frame also works on iOS.
Hope this helps somebody...