Oooh, now you're making life difficult!
So we've done some investigation and it turns out - it's not a bug and it's always been this way.
Are you ready for an adventure?
Having looked at the vyrelords, it turns out they are quite a unique NPC in that they have a field of view of 180 degrees. This means they can only see players that are front of them (from right to left of the way they're facing). That NPC that you noticed that wasn't aggressive was because it was facing away from you; as soon as it turned you can see it then becomes aggressive.
To clarify further that this hasn't changed, we managed to find a video from 5 years ago that shows exactly this: https://www.youtube.com/watch?v=FfmsG6sMsac&t=206s
They even comment how it doesn't have aggro and you need to use Corruption and Dragon Breath!
Hope that explains it! You're really testing us now, and unveiling some real fascinating facts about some RuneScape mobs.