There’s either an instance pupulation limit or a limit to the number of instances which can run parralel to each other. In other words, sometimes you won’t be allowed to enter the rift portal (stage 2). The game will not create additional phases to accommodate all players.
If you don’t enter on time, the mob will die and you won’t be able to earn credit. If you try to reset instance (player portrait function), you’ll eventually enter an empty portal with boss intact. Objective tracker is showing but you won’t earn loot or credit after killing it alone.