Thanks for your reply. I think you find the problem. There are two layers as basemap in this webmap. VectorTile_6191 is vector tile basemap from Esri, the other one (wms_1935)is a wms service which I used as a reference layer. As you mentioned, the problem occurs when there are two basemap layers in one webmap. After I removed one of layers, the button worked properly! However, it was fine in Journal, still messed in Series. Although there is only one basemap in webmap, the switch button keeps removing any layers above. Still struggling on that.