Thanks for your interest in Syncfusion products.
We are able to reproduce the mentioned issue in our previous version of Xamarin (iOS). We have already fixed this issue in SfGauge control (iOS)and the related sample with custom assemblies for the fix can be downloaded from the below link.
Sample Link: Xamarin.iOS_Binding_Modified.zip
This Issue fix will be available in our upcoming Xamarin SP release which is expected to be rolled out by the end of this month.
If the provided assemblies does not resolves your issue, please revert us back by modifying the provided sample to reproduce the issue and its replication procedure along with screenshots (if possible)? It will be very helpful for us to analyse on it and provide you the possible solution.
Please let us know, if you have any queries.