Major Bug On Baal And Similar Mobile Defense Missions:

[TraumaHunter]

There is a bug with Baal/similar mobile defense missions.  If the extraction is 1-2 tiles away from  the room you must defend - the mobs will spawn from extraction and run all the way until they hit the fan... which they can't destroy because they aren't programed to.

 

Thus you have to have half your team sit in the outdoor extraction area killing and kill the other half that spawns from the other direction and defending the objective if you wish to farm for enemies.

This results in people wondering why no enemies are coming when sometimes they have HUNDREDS of enemies coming at them.

(this is because the logic for mobile defense missions is basically spawn enemies 1-2 rooms away infinitely as mobs die)  
I can provide screenshots /video if needed.   It's VERY easy to replicate though... I'm sure you understand without me providing them.

[Tenebraex]

Confirmed, though the number of enemies spawning in that room may be a bit exaggerated  Even with the grate broken many enemies still cannot jump into the vent and will continue running into the ledge.