It's part of a promotion called What The Fanta in which people had to guess the flavour. I finished the post stating that I believed the flavour I had was green apple.
Since writing the post, I've had more sips of the drink. I kind of still believe that there is a hint of apple, but another suggestion has entered my consciousness.
I've noticed that there is an after taste of pear when I've tried it. Therefore, upon further review, I am not completely sure it's green apple.
It could be pear...