在使用 MapContext.addGroundOverlay 添加多个图层后，后加载完成的都会被覆盖，设置属性 zindex也不生效，是什么原因？

this.mapContext = wx.createMapContext('mapContent');

this.mapContext.addGroundOverlay({
	id: 1000,
	src: 'xxx',
	bounds: {
		// 西南角经纬度
		southwest: {
			latitude: 40.89595,
			longitude: 113.1608156
		},
		// 东北角经纬度
		northeast: {
			latitude: 40.91118,
			longitude: 113.17452
		}
	},
	visible: true,
	zIndex: 99,
	opacity: 1
});

this.mapContext.addGroundOverlay({
	id: 2000,
	src: 'xxx',
	bounds: {
		// 西南角经纬度
		southwest: {
			latitude: 40.87595,
			longitude: 113.1608156
		},
		// 东北角经纬度
		northeast: {
			latitude: 40.91518,
			longitude: 113.17452
		}
	},
	visible: true,
	zIndex: 9,
	opacity: 1
});