(UWP, 100.12 & 100.13)
Some polygons don't display the label that is specified on the overlay.
I have extracted some code to a demo project that shows the problem. I have added three polygons to an overlay and added a symbol renderer and a label definition. The large polygon doesn't show the label.

Code extract (see full demo in attached zip):
testLayer = new GraphicsOverlay
{
Id = "TEST_LAYER",
RenderingMode = GraphicsRenderingMode.Static,
LabelsEnabled = true,
};
//Symbol renderer
testLayer.Renderer = new SimpleRenderer(new SimpleLineSymbol
{
Color = Color.Yellow,
Style = SimpleLineSymbolStyle.Solid,
Width = 2
});
//Label def
var label = new LabelDefinition(new SimpleLabelExpression($"\"Test\""), new TextSymbol
{
BackgroundColor = Color.Aqua,
Color = Color.Black,
Size = 14
});