Hi,
with the newest Xamarin Forms version 5.0.0.2515 and the current syncfusion version 20.2.0.43 the watermark is no longer displayed.
The same issue appeared in Xamarin Forms Textinputs: https://github.com/xamarin/Xamarin.Forms/issues/15435 .
Adding
global::Xamarin.Forms.Forms.SetFlags("Disable_Accessibility_Experimental");
to the Mainactivity as suggested fixes this Issue in sfnumerixtextbox aswell so that might be related.
Kind regards