Hello Tomasz,
This is a know limitation for WMS and MapImageLayer. This visual limitations just happens if user tilt that the far away hills/mountains get seen.
Workaround: Set the tilt constraints to avoid that the user can tilt to this point where you can see this effect
SceneView | constraints
Example:
var view = new SceneView({
container: "viewDiv",
map: map,
constraints: {
tilt: {
max: 60
}
},
});
This technical limitation would be solved in the future.
Regards,
Sascha