MBTile render artifacts

I am using MBTile data file served with GeoServer.
In Cesium I use a WebMapServiceImageryProvider to query the layer. When rendering the layer it shows white blocks above (higher latitudes) the queried bbox.
How can I get rid of this?

I can create a geotiff file for the highest zoom layer, but I would like to use the MBTile data, because of the different presentation of the data in the different zoom levels (without the white blocks)