When the value of Text is set in XAML, Text value is ignored on Android. Use this example:
<telerikInput:RadAutoComplete x:Name="autoComplete" Text="Initial Text" Watermark="watermark" />
Observe that only the watermark appears (note: this issue happens with or without watermark).
Workaround:
Set the value of Text after view has loaded, for example in OnAppearing:
protected override void OnAppearing()
{
base.OnAppearing();
autoComplete.Text = "OnAppearing";
}