I can confirm it took me no more than 30 mins to acquire it in the same spot this evening, thanks for the tip!AK 5 unlocked today. Got the fire hidden appearance after farming the Suramar mobs at Moon Guard Stronghold for at most 30 minutes. I stayed near the first level and didn't go too far into the stronghold.
I'd spent about 3 hours farming the mobs in Suramar city, the insta spawn loyalists in the past having heard they dropped it, to no avail.