ReactiveUI.Fody | Fody extension to generate RaisePropertyChange notifications | iOS library

Binding the validation to a control template is currently not supported but you can show a validation error message in the view if you bind it to a seperate textblock (Documentation).

If you just want to use the MaterialDesign HelperText to show an error message, you don't need to use ReactiveUI.Validation but could do something like this:

Is this expected behaviour?

It is. In line with ObservableCollection only changes to the collection (or stream) are monitored.

Of course, what you want is quite common and, luckily, easy to achieve. From the ReactiveUI documentation:

DynamicData supports change tracking for classes that implement the INotifyPropertyChanged interface — ReactiveObjects. For example, if you'd like to do a WhenAnyValue on each element in a collection of changing objects, use the AutoRefresh() DynamicData operator

Simply add the AutoRefresh operator to the pipeline and use the Transform operator overload that accepts a flag to set transformOnRefresh to true.