Maven invitation: The feared - no reward/progress - 1 monster remaining?

This is not a bug. If you aren't present for the boss kill, you don't get credit (and no special drops). This is consistent with all bosses in the game.

If you die during the fight, your friend needs to wait for you to return, or another option is you can just lie there dead while he kills it. This is how all bosses in the game work.

The 1 monster remaining is the Maven, who does not die.