The dragon definitely resets if aggro swaps enough. Pretty sure it uses the same mechanic as the jungle mobs. Why is the big important dragon the only one without a patience bar? I've actually been trolled by this once when an enemy team backed off the dragon, I took aggro, and it simply reset. Should have been ours. Unlucky. This caused my team to split up as one of our members did a pretty poor hard dive then we lost everything.
I always never got why dragon didn't have one. Maybe it's to discourage highly organized team from going so close to the patience limit to discard the drake attempt easily. Doubt it tho.
Same with rift herald maybe.