This matter was already explained in due course, but it does not hurt to remember it...
The authentic flag of Gilneas is represented by the Greymane family brand:
Why it does not appear in WoW then? What happened? Well, the same as always: Blizzard consciously or unconsciously loves to ignore the lore established, and does not hesitate to improvise and create nonsensical changes when they want.
Remember that for Blizzard the Gameplay (more economical income) is always more important than the Lore (more quality games).
So, any of us have more knowledge about the lore than the whole team of Blizzard, luckily occasionally, a guy with a red shirt dares to expose Blizzard, demonstrating their manifest incompetence at the time to create quality stories.
However, as always, Blizzard is bound to rectify, and therefore they had to include the Greymane family brand in the Racial Crest previously shown.
Finally remember also that ignoring their true flag is not the only humiliation suffered by the Kingdom of Gilneas. A picture is worth a thousand words: