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I test your demo, I get the same issue, And I find this issue was reported in the github, you can waitting for update in github thread.
And I test with MVVM demo from xamarin forms, it worked as normal. Here is blog and demo thread.
you used mvvmcross, this is a third-part plugin, you can open a issue in StackOverflow, Add add tags to at least MvvmCross and Xamarin, MVVMCross supporter will check it.
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