Invite only not working

[Maqabir]

I was chasing wisps and managed to get a few when suddenly a whole bunch of people join my Invite only game and refuse to let me extract.

Seeing as it's been stated that progress is saved in the plains I tried extracting and lost all my wisps and other items gained so far.

Please let us extract individually or let us at least make our games private.

[CxLL]

Can confirm.
Seems to be a rather old issue actually, maybe about a year or so. Got resolved at one point, for a moderate amount of time, specifically until Update 22.
The chat /join command seems to be usable regardless of the player matchmaking settings. What I've been told back when I've first complained is that people who have added you to the friend list and are currently in the "pending" zone actually have the ability to exploit your "friends only" setting. However, this has indeed happened while my preferences were set to invite only.
Now, this might be closely related with the workings of the Plains themselves, as it's seemingly a special type of instance. Will see to test it a bit today.
Should anything interesting arise, I'll update this note or reply to the thread once more.