I'm sorry to hear you all are having this issue! It's been reported to the team who are investigating. Please try steps like joining another player's game and looking for Hayes there. Let me know if you have any success with this, and I'll pass that on!
Same issue here, did the Lincoln Memorial mission with matchmaking (we were 2 players) at the end of the mission the other player left the group, I went to the campus to speak with Henry hayes and can't.