I noticed that some Unicode characters display a box with an x through it. For example, the ễ character in the image below.

This is coming from a JSON feed. The character is correctly interpreted and displayed in the string output in the console (as shown in the image below)

I don't know if the limitation is because of the system font (I'm using "font:MediumBoldSystemFont") or because encoding or what?
Any help is much appreciated.