I found a few big bugs on new map viewer, in last several years, ESRI have not fix yet.
In 2024, I just found another big bugs, map viewer has been out for over 10 years, it should not have such big bugs today.
That is why many people still use classic, not use the new one. Too much hidden bugs.
Today's bugs is when feature layer is from uploaded file geodatabase, new map viewer failed to show you the button to change symbol color and failed to show the button to add label.
The classic map viewer has all the button for you to change color, label it for those file geodatabased based feature layer.
classic map view, you can change color, add label, while new map viewer you can't, it is big bugs.
if the feature layer is not from uploaded file geodatabase, then map viewer works fine, allow you change color and add label.
