Character Not Displaying Correctly

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

[upload|DmMGMuMMTj5yBx0/0P/BYw==]

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)

[upload|1k4jsik4kD9305mTERdteA==]

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.

http://graphemica.com/%E1%BB%85