Not sure if this is an oversight but since the season 2 update, the game has consistently been doing odd things regarding party options. The biggest being that my party is set to invite only or close...
Ideas
Today, one of our players discovered that this bug is a consequence of our settings for who is allowed to join our party. I have had extra players in my challenges for WEEKS and had no clue why, until today, when one of our players switched from Friends-Can-Join to Nobody-Can-Join and immediately began encountering the same bug. Since switching back to Friends-Can-Join, none of my challenges have bugged.