I can answer that since I did a lot of testing when it comes to proximity dialogue and it's not about how they're chained.

Basically the NPCs have a radius around them for proximity dialogue. As soon as you enter their circle or if you're already in it, you'll be pulled into a cutscene. However some NPCs such as Mayrina or Nadira (Tiefling Soul Coin lady) exhibit a minimal radius in which they will ignore you if you are too close to them and instead pick another person.

To better illustrate this, here is a visualization of the issue;

[Linked Image from i.imgur.com]

And here is a video showcasing this radius;



Notice that in the video my main character is next to Mayrina, Lae'zel is further away and Shadowheart is standing next to the Possessed Gnome lady that we save (you can see her on the minimap). However Mayrina will ignore my main character and the Possessed Gnome lady will ignore Shadowheart. Instead she pulls my main character from across the room into a conversation and Mayrina pulls in Lae'zel.

In OP's case however, Mirkon strolled by their main character because Mirkon usually runs away in that quest and he won't trigger a proximity dialogue if he is too far away from his original location. However once he starts walking back and eventually gets there, he will pull in whoever is closest to him. Which is also something I've experienced.