If you are rendering your label in a JS 3.x map (like the current Map Viewer), TextFormatting.NewLine and "\n" are both ignored.
In a JS 4.x (Map Viewer Beta, certain Configurable Apps), both are valid ways of adding a new line.
Here's a layer of ours with multiline labels, first in a JS 3.x map:
![jcarlson_1-1614000666436.png jcarlson_1-1614000666436.png](https://community.esri.com/t5/image/serverpage/image-id/6556i3EB88C156EBC8D8D/image-size/medium?v=v2&px=400)
And then in JS 4.x:
![jcarlson_2-1614000773967.png jcarlson_2-1614000773967.png](https://community.esri.com/t5/image/serverpage/image-id/6557i4FC0460144A4C217/image-size/medium?v=v2&px=400)
What version of Portal are you on? If you're at 10.8 or later, you can also create Hosted Map Image Layers that can take advantage of all the advanced labeling and symbology of Pro, but are based on a hosted feature layer. Works just the same in either map viewer, too.
- Josh Carlson
Kendall County GIS