So with the minichamps they have like Range 30 listed on the idols.
If you run too far from the idol, and have monsters hot on your tail after a certain point they get "leashed" and teleport back within their intended range.
Sometimes however they don't teleport back, and they get stuck outside of the range they're supposed to be within.
When the minichamps end it cleans up all active mobs, and puts the idol to inactive.
With the Wyvern [Renowned] for instance I had lured 5 fairy dragons (I almost died) south of the spawn, and east over the bridge towards the Devourer of souls [Renowned] minichamp which is outside of their "leash" range.
The only time I've had the champions not reset properly is when there's monsters outside of the intended range.
When I was fighting the grey goblin renowned (and the other renowned that spawns with him) monsters ran from me when they got low life, and may have left the spawn area causing it to happen there as well.