Twice it has disappeared and forced me to start over. Is this a new thing where you fail the mission? if so can you add some text indicating you have failed. This never happened before. normally it stops at the gate, but i have watched it disappear 2 times now.
This started to happen sometime ago, I'm not sure which update it was but we have had several updates since and it's still occurring.
The best solution is to concentrate fire on it to stop it before it can reach the gate, then it is forced to stay where you can interact with it at the end of the combat.