Polygon opacity issue when dragging the map

[aem34]

Hello,
After having created a polygon,
whenever i drag the map around or i remove and create a new polygon
with the (theorical) same opacity, the polygon is changing its own opacity
towards something less transparent at each move.

Here is my code :
var polygoneParcelleHeig = map.drawPolygon({
paths: polyLineArray,
strokeColor: "#0FF000",
strokeOpacity: 0.8,
strokeWeight: 2,
fillColor: "#0FF000",
fillOpacity: 0.5,
clickable: false,
draggable: false,
editable: false,
geodesic: false
});

is there something wrong with my parameters please ?