In 28.03.2011 I’ve achieved an exalted reputation with the Netherwings (I even have an achievement) and was able ti but the drakes. Now, my repution with the netherwings is neutral, I can’t buy the drakes, and have to farm the rep again…
according to armory you got exalted with netherwing on 28/03/2011 
You sure you didn’t do it on another character? Old reps aren’t shared, unfortunately.
I just got exalted too, and my god the speed of that ride back to Shattrath. Images of skeleton on the back of a Netherwing come to mind.
Totaly sure. 
I even remember do it it on this character. So time ago, they’ve revamped th whole island, and quests, so maybe then my rep got zeroed.
https://worldofwarcraft.blizzard.com/en-gb/character/eu/burning-legion/carelion/achievements/reputation/the-burning-crusade
I had a similar thing, when i first turned in a quest after the anniversary update i saw “guild reputation increased by 150”
I was like wtf that was like maxed 15 years ago 