TextFormatting.NewLine will only work with 4.x of the JavaScript API. The current Map Viewer, along with many apps including Web AppBuilder and as of 2019 Operations Dashboard, are written in 3.x. In these apps new lines in labels are not honored.
However, the Map Viewer Beta, along with Experience Builder and 'newer apps' are written in 4.x and therefore honor new lines. The Map Viewer Beta does not have an option to configure labels - it is currently on the roadmap. In the meantime, you can configure labels with new lines in the standard Map Viewer. They will not display in the Map Viewer as it is written in 3.x. However they will be saved in the Web Map and when you view the Web Map through a 4.x app the labels will appear as you have configured.