The rendering issue is fixed in the Map Viewer Beta release. Refer to https://www.esri.com/arcgis-blog/products/arcgis-online/mapping/map-viewer-beta-coming-soon/.
Quick steps to open a web map in Map Viewer Beta:
1. In the Content page, click on the web map to see item details.
2. On the right side of the page, there is an option to open the web map using Map Viewer Beta.