Picker on iOS - some Pickers update as items scrolled, some don't

We have two Pickers on a Page:

<Picker
    AutomationId="IdOwnerOfVehiclePicker"
    Margin="0,0,0,10"
    x:Name="OwnerOfVehiclePicker"
    Title="Please select"
    ItemsSource="{Binding OwnerOfVehicleOptions}"
    ItemDisplayBinding ="{Binding Description}"
    SelectedItem ="{Binding SelectedOwnerOfVehicle.Value}"
    Style="{StaticResource PickerStyle}"
    >
    <Picker.Triggers>
        <DataTrigger 
            TargetType="Picker"
            Binding="{Binding SelectedOwnerOfVehicle.IsValid}"
            Value="False">
            <Setter Property="behaviours:LineColorBehavior.ApplyLineColor" Value="True" />
            <Setter Property="behaviours:LineColorBehavior.LineColor" Value="{StaticResource ErrorColor}" />
        </DataTrigger>
        <DataTrigger 
            TargetType="Picker"
            Binding="{Binding SelectedOwnerOfVehicle.IsValid, Converter={StaticResource InverseBoolConverter}}"
            Value="False">
            <Setter Property="behaviours:LineColorBehavior.ApplyLineColor" Value="True" />
            <Setter Property="behaviours:LineColorBehavior.LineColor" Value="{StaticResource BackgroundColor}" />
        </DataTrigger>
    </Picker.Triggers>
    <Picker.Behaviors>
        <behaviours:EventToCommandBehavior
            EventName="SelectedIndexChanged"
            Command="{Binding ValidateSelectedOwnerOfVehicleCommand}" />
    </Picker.Behaviors>
</Picker>
<Label 
    Text="{Binding SelectedOwnerOfVehicle.Errors, Converter={StaticResource FirstValidationErrorConverter}}"
    Style="{StaticResource ValidationMessageStyle}">
</Label>

<Label 
    Margin="0,10,0,0"
    Text="Reason For Cover"
    Style="{StaticResource FormLabel}" />
<Picker
    AutomationId="IdReasonForCoverPicker"
    Margin="0,0,0,10"
    x:Name="ReasonForCoverPicker"
    Title="Please select"
    ItemsSource="{Binding ReasonsForCoverPurchaseOptions}"
    ItemDisplayBinding ="{Binding Description}"
    SelectedItem ="{Binding SelectedReasonForCoverPurchase.Value}"
    Style="{StaticResource PickerStyle}"
    >
    <Picker.Triggers>
        <DataTrigger 
            TargetType="Picker"
            Binding="{Binding SelectedReasonForCoverPurchase.IsValid}"
            Value="False">
            <Setter Property="behaviours:LineColorBehavior.ApplyLineColor" Value="True" />
            <Setter Property="behaviours:LineColorBehavior.LineColor" Value="{StaticResource ErrorColor}" />
        </DataTrigger>
        <DataTrigger 
            TargetType="Picker"
            Binding="{Binding SelectedReasonForCoverPurchase.IsValid, Converter={StaticResource InverseBoolConverter}}"
            Value="False">
            <Setter Property="behaviours:LineColorBehavior.ApplyLineColor" Value="True" />
            <Setter Property="behaviours:LineColorBehavior.LineColor" Value="{StaticResource BackgroundColor}" />
        </DataTrigger>
    </Picker.Triggers>
    <Picker.Behaviors>
        <behaviours:EventToCommandBehavior
            EventName="SelectedIndexChanged"
            Command="{Binding ValidateSelectedReasonForCoverPurchaseCommand}" />
    </Picker.Behaviors>
</Picker>

The OwnerOfVehiclePicker does not update its value as the available value list is scrolled - only when the Done button is tapped.

The ReasonForCover does update its value as the available value list is scrolled - we don't have to tap the Done button.

I cannot see any difference between these two. I don't think the Triggers are relevant - but I've left them in the XAML above, in case someone can spot a side effect.

Can anyone suggest why their behaviour would be different? This is breaking my UITest logic for selecting Picker values.