[Known] Doubled Zombies when Creating a Party

Ran into this one today, and unfortunately didn't have the time to take a screenshot. I'll describe the situation as best as I can, and try to replicate it if possible and get back to you folks.

I was in the Prisoner's Gate with my maximum number of zombies out (6) when I created a party with someone. When he came into the instance, my number of zombies instantly doubled to twelve as I was standing around waiting for him to show up. The extras quickly died off to the next monsters I found, and when I tried summoning more I couldn't breach my cap again.

Not sure what might have caused it, and I hope what little I could tell you helps. If I run into it again, I'll know what to do next time so I can submit a proper report.