Okay so this has been an issue for a very long time and frankly I'm a bit stumped but want to at least record this.

When logging in for the first time with a brand new character (or on first install of the addon I assume) the chat font fails to update despite the function clearly running (tabs are styled as is edit box).

The default font size is 14. Reloading the UI doesn't apply the font.

Change the font size to 15 and /reload. Font is applied.

Change font back to 14 and /reload. Font is still applied properly and works at any size and on that character permanently going forwards (or until WTF folder is removed).

It's not the biggest issue in the world but definitely has a bad first impression and I wonder how many players never change the font size and as such have no idea that this occurs.