set.Bind (TxtActivityText).For(txt => txt.AttributedText).To (vm => vm.Activity).WithConversion("ActivitySpanMessage");

set.Bind (TxtActivityText).For(txt => txt.Text).To (vm => vm.Activity).WithConversion("ActivitySpanMessage"); // Works fine also binding to the default property

TxtActivityText.AttributedText = SpanMessageHelper.ConvertToAttributedString(ViewModel.Activity, null, null, null); // This function is the same I call in the ActivitySpanMessageConverter