Sometimes when the objective is to lure out an ambush, either surprise objective or stated, if the ambush is put down quick enough the “Objective” target (radiating circle) doesn’t go away. The “Open Portal” button is available but the map indicates there’s still something to complete. There is no mob there still to defeat.
Completed Objective Not Clearing Objective Target Area
This may be related to the “Seal the ‘random’ Gate” objectives that are completed with the “Open Portal” button available but mobs are still appearing from the gate.