SKGLView clears canvas between every Dispatcher.BeginInvokeOnMainThread

In a Xamarin Forms app i use a SKGLView to write spokes to a circle via the GPU, the thing is the whole canvas gets completly redrawn(cleared) with every call to
Dispatcher.BeginInvokeOnMainThread(() =>{SKGLView.InvalidateSurface(); }); not just update the spoke of 10 degrees.
But if i use a SKCanvasView instead with the same code the canvas keeps the rest of the circle and updates the spoke as expected.
The Dispatcher.BeginInvokeOnMainThread(() is called from an SignalR.On event.
All this is done on an Android emulator.
Thx Peder