When forming a new "Open to All" party (usually for prestige missions) it seems the party disappears from the party finder after a short while. In this limbo state , no random people can join my party either by joining a party from the mission console or the party finder.
I can test if my party is available by opening the mission console from inside the lobby and trying to "Join" a party for the mission I am currently in. If my party is publicly available the "Start" button just greys out with no dialog appearing. If my party is in the limbo state I get the "Sorry, we could not find you..." message.
The limbo state seems to occur about 1-2 minutes after forming the party. I have waited an extended amount of time and the party never comes back out of limbo. Toggling between "Open to All" and back again in the party settings seems to have no effect.
This makes it extremely annoying when trying to form a party. Most people are probably unaware of this and wondering why it is so hard to form a group.
"I can test if my party is available by opening the mission console from inside the lobby and trying to "Join" a party for the mission I am currently in"
Are you trying to join yourself? That's not a very good method of testing things.