I literally just finished Destroids Rise Again around 15 minutes ago, joining the Open Mission in time to personally destroy two Retribution Devices and fighting for the entire duration of the Mega-Destroid fight. I did not receive the "Open Mission: Destroids Rise Again" Perk, nor the Steam Achievement.

Did I mess up by not being there from the beginning? I received enough credit in the fight to be ranked 3rd in the list, at triple the points of the 4th guy, and roughly 70% of the points of the 2nd guy.

Addendum: That was on Downtown #8, not in a party, with 5 of us in total fighting the Mega-Destroid.