Icons displayed as "write to us" in Oracle APEX

Recently, I encountered something strange when working with icons. To be more precise, I was implementing a Interactive Grid with a customized column (HTML Expression).

Pretty simple, right? Unfortunately, when I run the page, my column did not have the "fa-user" icon, instead there was a "write to us" text:

I was confused that at first, I questioned, to whom shall I "write". Is it a license issue on Font Awesome or Oracle APEX? I did some online search and found nothing. Then, I decided to compare my column having this issue to the other columns that are OK. As it turned out, the issue was related to my span was missing class "fa" even if I already have "fa-user":

Now, the mystery is resolved! My icons are back again as intended: