I found it bugged when I received invites when I was in the tower mostly. I'd try to accept it and it would say I can't join due to player activity (even though it also clearly shows the player inviting, in orbit.) And so far only invites through the Apps lfg function have caused this for me. The only fix I've had is to be in orbit when receiving the invite.

First, check your Roster Preferences settings on the Roster screen. It’s possible you accidentally flipped the switch for [b]Fireteam Privacy/invites[/b] to [i]Closed[/i] or changed [b]Invite Notifications[/b] to [i]Hide All[/i]. Second, check your Privacy settings on your given platform and in the D2 app to make sure they aren’t preventing you from receiving invites. (΄◉◞౪◟◉｀)